Injury risk associated with cannabis and cocaine use

Drug and Alcohol Dependence 72 (2003) 99–115

Review

Injury risk associated with cannabis and cocaine use夽 Scott Macdonald a,∗ , Kristin Anglin-Bodrug a , Robert E. Mann b , Patricia Erickson b , Andrew Hathaway b , Mary Chipman c , Margaret Rylett a a

Center for Addiction and Mental Health, 200-100 Collip Circle, Suite 200, London, Ont., Canada N6G 4X8 b Center for Addiction and Mental Health, 33 Russell Street, Toronto, Ont., Canada M5S 2S1 c Department of Public Health Sciences, University of Toronto, Toronto, Ont., Canada M5S 1A8 Received 23 January 2003; received in revised form 19 June 2003; accepted 25 June 2003

Abstract The purpose of this paper is to review the results and limitations of studies of injury risks associated with cannabis and cocaine use. Three types of fatal and non-fatal injuries are considered: injuries due to collisions, intentional injuries and injuries in general. Four types of studies were reviewed: (I) laboratory studies, (II) descriptive and analytic epidemiological studies on the prevalence of cannabis or cocaine use through drug testing of those injured, (III) studies of non-clinical samples, and (IV) studies of clinical samples of drug users. The research that utilized drug tests showed similar proportions testing positive for cannabis in fatal and non-fatal injury groups, and for collisions, violence and injuries in general. By contrast, large differences in the average proportions testing positive for cocaine were found among these same injury groups. For example, 28.7% of people with intentional injuries (primarily homicides) tested positive for cocaine, while 4.5% of injured drivers tested positive. Studies of non-clinical samples have shown that both cannabis and cocaine use are related to intentional injuries and injuries in general. Results indicate higher risk for all types of injuries among cannabis and cocaine clients in treatment. Strengths and limitations of the different types of studies are discussed. More rigorous studies are needed which should focus on ruling out alternative explanations for relationships between drug use and injuries. © 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Cannabis; Cocaine; Injuries; Accidents; Violence; Motor vehicle

1. Introduction Cannabis and cocaine are two of the most commonly used illicit drugs (e.g. Adlaf et al., 1994) and the hazards posed by these drugs to users have been the subject of considerable debate over the years (e.g. Alexander, 1990; Commission of Inquiry into the Non-Medical Use of Drugs, 1973; Erickson, 1980). A major concern about the use of any psychoactive drug is its possible influence on risk of injury from various causes, including motor vehicle collisions and violence.

夽 The views expressed in this paper are those of the authors and do not necessarily reflect the views of the Center for Addiction and Mental Health. ∗ Corresponding author. Tel.: +1-519-858-5000x22006; fax: +1-519-858-5199. E-mail address: [email protected] (S. Macdonald).

The purpose of this paper is to review the available empirical research in order to assess the risks that cannabis and cocaine may pose for traffic collisions, intentional injuries and injuries in general. The review also aims to determine whether risks of cannabis or cocaine use are the same across these injury groups and between fatal and non-fatal injuries within each injury group. The review also permits an analysis of the relative dangers of each drug. The strengths and limitations of these studies and priorities for future research are identified. For this review, computerized literature searches were conducted using keywords such as cannabis, cocaine, violence and injuries, with search programs (e.g. PubMed and PsychoINFO) to identify published articles. In addition, manual searches of selected journals were conducted where articles on this topic are likely to be published. Bibliographic references of recent relevant papers were also reviewed to ensure that the widest range of suitable studies

0376-8716/$ – see front matter © 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/S0376-8716(03)00202-3

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was included. Inclusion criteria were studies that specifically addressed the relationship between cannabis or cocaine use/abuse and injury risk. Studies that investigated the risks associated with cannabis and cocaine use/abuse and driver-related traffic injuries, intentional injuries and injuries in general were included in this review. These studies are further sub-divided into fatal and non-fatal injuries. Studies of other types of specific injuries, such as those occurring in the workplace, are excluded. The literature on domestic violence, child abuse, physical threats and fights not involving injuries requiring hospitalization is not addressed in this review. Some traffic and intentional injuries were included under injuries in general if these groups were not analyzed separately. In order to obtain an adequate understanding of the influence of cannabis and cocaine on injury and collision risk, laboratory studies, epidemiological studies and survey studies are necessary. In the past two decades, numerous epidemiological studies have been published on the involvement of these drugs in injuries and collisions. In this review, conclusions will be based on four types of studies: (I) Laboratory studies on the direct effects of cannabis or cocaine on psychomotor performance; (II) descriptive and analytic epidemiological studies on the prevalence of cannabis or cocaine use through drug testing in injuries; (III) studies of self-reported drug use and injuries in non-clinical samples; and (IV) studies of clinical samples of drug users. The findings of these studies, and their strengths and limitations will be addressed.

2. The psychomotor effects of cannabis and cocaine In order to better understand how cannabis and cocaine use might be related to injuries, it is useful to study the direct effects of these drugs on psychomotor performance. Numerous laboratory studies and reviews of these studies have been conducted. Conclusions from these reviews are described briefly. Laboratory research indicates that cannabis impairs various behavioral and cognitive skills, including those related to safe driving. In a landmark review of cannabis studies, Moskowitz (1985) concluded that cannabis use impairs driver performance under a variety of experimental conditions. Berghaus and Guo (1995) conducted a meta-analysis of experimental studies and concluded that smoking marijuana causes impairment of every performance area connected with safe driving of a vehicle, such as tracking, psychomotor skills, reaction time, visual functions, and attention. Of these performance measures, the most deleterious effects of cannabis use were found for attention, tracking and psychomotor skills (Berghaus and Guo, 1995; Coambs and McAndrews, 1994). Aside from these direct effects, cognitive functioning could be affected by the duration of drug use (Hall and Solowij, 1998). Also, the potential withdrawal effects of

heavy, long term cannabis use such as restlessness, insomnia, and anxiety could influence injury risk (Ashton, 2001) or aggression (Kouri et al., 1999). Cocaine stimulates the central nervous system; therefore, it may not be surprising that most laboratory studies have failed to find deficits in the performance of simple tasks following consumption of this drug (Ferrara, 1987), while in other instances performance improvements have been reported (Coambs and McAndrews, 1994; McKim, 1986). However, the effects of cocaine are not unidimensional in that the effects of over-stimulation on performance may be qualitatively, as well as quantitatively, different from the effects of mild-to-moderate stimulation (e.g. Burns, 1993a,b). While small doses of cocaine may improve psychomotor performance, larger amounts taken over longer periods may impair performance due to dependence and withdrawal effects (Bolla et al., 1999; Burns, 1993a; O’Hanlon and de Gier, 1986; Wolschrijn et al., 1991). Since this review also addresses the relationship between cannabis/cocaine use and violence, the laboratory research literature on how these substances might affect aggression is relevant. The experimental studies on cannabis and aggression have not clearly demonstrated a causal link between the two and some findings are contradictory. For example, one study found increased aggressive responses in the first hour after smoking (Cherek et al., 1993). Another study indicated that those who received high doses of tetrahydrocannabinol (THC) were less likely to respond aggressively than those who received low doses (Myerscough and Taylor, 1985). Similarly, laboratory research has not conclusively shown that cocaine increases aggressive tendencies (McKim, 1986), although some studies have found that subjects in cocaine conditions reacted more aggressively than controls (Licata et al., 1993). The most typical responses to cocaine ingestion are improved mood and a reduction of fatigue (McKim, 1986). In general, laboratory studies have found that the ingestion of cannabis is related to performance deficits, but no similar relationship has been found for cocaine. These studies are highly useful for determining the pharmacological effects of drugs, but the results lack external validity and cannot be directly generalized to real world conditions. Specifically, the research methodology in laboratory studies often involves completing reaction time or other cognitive tasks to the best of one’s ability. As a result, these studies are more likely measuring the effect of drugs during peak performance as opposed to typical performance. It seems plausible that, in real world situations, users of these drugs might rarely put themselves in situations where injury risk is increased or they might compensate for their impairment (Smiley, 1999). Moreover, experimental studies have often administered low doses of cannabis and, thus, the results are not indicative of driving patterns of heavy cannabis users (Crowley and Courtney, 1999; Maes et al., 1999). These studies also do not adequately address the impact of long term use and abuse of drugs.

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3. Studies using drug tests of injured subjects to detect cocaine or cannabis metabolites Studies that obtained drug tests of urine or blood from those injured at the time of their injury are included in this section. This section is divided into three parts, epidemiological studies of those injured in car collisions, intentionally and due to all causes. These studies were further subdivided into fatal injuries and non-fatal injuries. The research methodologies and results of these studies are described in Table 1. Studies on driver injuries (i.e. fatal and non-fatal driver injuries) are shown in Part A, studies on intentional injuries are presented in Part B, and general injuries are presented in Part C. A large number of descriptive studies have been conducted where the blood or urine of injured people has been analyzed for the presence of cocaine or cannabis metabolites. Twenty-five studies were found on injured drivers, eight studies on intentional injuries and 14 studies on injuries in general. To better understand variations in the prevalence of drug metabolites for different injury contexts, the range (i.e. studies with the highest and lowest estimates of the percent testing positive for cannabis and cocaine) and the real mean of all studies (i.e. the total number testing positive divided by the total number of subjects) are described below and are presented in Figs. 1 and 2 . Some studies were excluded from these figures if the sample was biased or if the true mean could not be calculated. For example, Williams et al. (1985) was excluded because the sample was restricted to males between the ages of 15 and 34, who do not reflect usage rates in the general population. Bailey and Shaw (1989) study was excluded because the sample included overdoses, which are not due to external causes. The McDonald et al. (1999) study, which found that 46% tested positive for cannabis, was excluded from this figure because it was conducted in Jamaica where usage rates for cannabis are extremely high. Other studies were excluded from the figures if only subjects who tested positive were included. Most studies that investigated the prevalence of cocaine and cannabis use through drug testing among patients requiring treatment in an Emergency Room (ER) or among those who were fatally injured did not require consent for their participation. Approximately 11% of the studies in the review required consent. In addition, few studies that use drug tests have control groups, thereby making it difficult to determine whether drug presence is a risk factor. The likely reasons that few studies include controls is that they can be difficult to access and that consent from this group is usually required. Consent is likely to discourage the participation of users more than non-users, which would translate into inflated relative risks or odds ratios. Some studies have used comparison groups of pedestrians, who are not appropriate for relative risks. Differences in subject participation, research methodology and jurisdiction might explain in part, the variation with prevalence rates across the studies.

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In the literature on impaired driving by alcohol, a consistent observation is found that a much higher percentage of those in fatal crashes were impaired by alcohol than those in non-fatal crashes. Recent crash data in the United States shows that about 41% of the fatal crashes have BACs of at least 100 mg%, while only 9% of crashes with non-fatal injuries have BACs at this level (National Highway Traffic Safety Administration, 2001). For the more serious injuries requiring treatment in an ER, the percent impaired is likely higher than those involved in collisions not requiring medical attention (Donelson, 1988). Given the very large number of epidemiological studies that have clearly demonstrated a causal link between alcohol impairment and collisions, this observation may have relevance for understanding the importance of cannabis or cocaine in collisions. Therefore, one comparison of interest is the average percent that tested positive for drug metabolites in fatal collisions versus those not fatally injured in collisions. The main strength of studies that use drug tests is that the data are free of the biases found in self-reports. However, in most studies, urinalysis tests were used, which are not accurate for determining whether those injured were under the influence of drugs at the time of the injury. Urinalysis test results cannot measure the degree of drug impairment or the method of drug use, only whether drug use occurred sometime in the past, up to many weeks for cannabis and 5 days for cocaine (Kapur, 1994). Blood tests that measure active THC levels are the most accurate measure of recent cannabis use and the best method to assess culpability (O’Kane et al., 2002). Unfortunately, active THC levels in blood samples can be distorted by several factors, such as the time interval from the sample to analysis (O’Kane et al., 2002). Blood tests are more intrusive and handling techniques are more delicate, which likely explains their limited use. Saliva testing is another approach that may have merit for detecting recent use, but further evaluation is needed (O’Kane et al., 2002). 3.1. Injured drivers There have been many epidemiological studies that have reported drug tests of fatally and non-fatally injured drivers. The percent of fatally injured drivers testing positive for cannabis (see Part A of Table 1) ranged from 1.4 to 27.5% (M = 7.8%); while for non-fatally injured drivers the percent ranged from 5 to 16.9% (M = 11.9%) (see Fig. 1). The percent of fatally injured drivers testing positive for cocaine ranged from 0.3 to 9.8% (M = 4.6%) and for non-fatally injured drivers ranged from 0.6 to 5.6% (M = 4.3%) (see Fig. 2). There is little difference in the percent testing positive between fatal and non-fatal driver groups for either cannabis or cocaine. Although many studies have been conducted on the prevalence of positive drug tests among injured drivers, few studies incorporated control groups so that assessment of relative risk could be estimated. The best methodological

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Table 1 Summary of study results on the percentage of injury patients testing positive for cocaine and cannabis Authors

Jurisdiction

% Positive cocaine

% Positive cannabis

Number and group injured

Fatal injuries Budd et al., 1989

USA

Cimbura et al., 1990 Crouch et al., 1993 del Ri´o and Alvarez, 2000 del Ri´o et al., 2002 Drummer, 1995a Dussault et al., 2002 Everest and Tunbridge, 1989 Fortenberry et al., 1986 Garriott et al., 1986 Gjerde et al., 1993 Logan and Schwilke, 1996 Mason and McBay, 1984 McBay, 1986 McLean et al., 1987 Mercer and Jeffery, 1995

Canada USA Spain Spain Australia Canada UK USA USA Norway USA USA USA Tasmania Canada

9.8 8.0 Not 8.0 7.4 5.2 0.1 6.8 Not Not Not Not 3.0 0.3 Not 2.2 4.0

19.6 18.5 10.9 13.0 1.4 2.2 11.0 19.5 2.8 10.8 27.5 5.0 11.0 7.8 13.4 9.5 13.0

Study 1: 102 drivers Study 2: 492 drivers 1169 drivers 168 truck drivers 285 drivers 5745 drivers 1045 drivers 354 drivers 744 drivers and motorcyclists 510 drivers 69 drivers 159 drivers 318 drivers 600 drivers 2610 drivers 42 drivers 227 drivers

Non-fatal injuries Kintz et al., 2000 Longo et al., 2000a Lowenstein and Koziol-McLain, 2001 Marquet et al., 1998 McLean et al., 1987 Peden et al., 2000b Soderstrom et al., 1995

France Australia USA France Tasmania South Africa USA

Stoduto et al., 1993 Sugrue et al., 1995 Terhune and Fell, 1982

Canada Australia USA

reported

9.6 10.8 16.9 13.9 5.0 15.7 12.1 Not reported 13.9 15.2 10.0

198 drivers 2500 drivers 414 drivers 296 drivers 37 drivers in accidents 72 drivers in collisions 331 drivers and motorcyclists 1338 drivers and motorcyclists 339 drivers 164 drivers 500 drivers

33.8 Not reported Not reported Not reported Not reported Not reported

130 homicides 223 homicides 179 homicides 145 homicides 324 homicides 2824 homicides

A. Studies of drivers

Not Not 3.6 1.0 2.7 2.0 Not 5.6 5.3 0.6 Not

reported

reported reported reported reported

reported

reported reported

reported

B. Studies of intentional/violent injuries Fatal injuries Garriott et al., 1986 Hanzlick and Gowitt, 1991 Haruff et al., 1988 McGonigal et al., 1993

USA USA USA USA

Tardiff et al., 1989

USA

Not reported 40.0 17.3 9.0 (1985) 39.1 (1990) 31.3

Non-fatal injuries Macdonald et al., 1999b Peden et al., 2000b Zautche et al., 1998

Canada South Africa USA

Not reported 2.7 20.9

13.2 34.2 4.0

62 patients with violent injuries 139 patients with violent injuries 582 patients with penetrating injuries drug-tested

USA USA USA USA Canada

27.4 Not reported 26.7 8.0 Not reported

Not reported 12.0 Not reported 11.0 3.9

3300 postmortem cases 500 postmortem cases 14 843 fatalities 160 fatalities 68 occupational accident fatalities

C. Studies of other injuries i. Fatal injuries Hood et al., 1990 Isenschmid and Caplan, 1988 Marzuk et al., 1995 Rivara et al., 1989 Shannon et al., 1993

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Table 1 (Continued ). Authors

Jurisdiction

% Positive cocaine

% Positive cannabis

Number and group injured

ii. Non-fatal injuries Carrigan et al., 2000 Clark and Harchelroad, 1991 Cornwell et al., 1998 Levy et al., 1996 Lindenbaum et al., 1989 Macdonald et al., 1999b Peden et al., 2000b Pinero-De Fuentes et al., 1998b Soderstrom et al., 1997b

England USA USA USA USA Canada South Africa Venezuela USA

3.0 21.0 31.0 22.0 54.0 Not reported 0.0 13.5 14

13.0 24.0 Not reported 21.0 37.0 6.3 31.4 7.4 10.9

79 patients 177 mildly injured patients 516 critically injured patients 628 orthopedic trauma patients 169 trauma patients 644 accidental injuries 43 trauma patients 148 trauma patients 709 trauma patients

a b

Consent needed for participation of control group only. Consent needed for participation.

studies are analytic epidemiological studies that utilize either the case-control method (Ferrara et al., 1990; Marquet et al., 1998) or methods used to ascribe crash responsibility (Drummer, 1995; Longo et al., 2000a; Lowenstein and Koziol-McLain, 2001) (see Table 1). Statistical significance was calculated in only one study for either cannabis or cocaine. In this case-control ER study conducted in France, urine tests were conducted for 296 injured drivers and for 278 control patients for the presence of cocaine and cannabis (Marquet et al., 1998). The study is unique in the field because consent was not required for urine tests, and therefore, the results are free of selection biases. No differences were found for cocaine; however, the sample sizes were small. All drivers testing positive for cannabis were no more likely than controls to be involved in collisions. However, when the analyses were restricted to women only, cannabis

was significantly related to collisions (Marquet et al., 1998). In a case-control study by Ferrara et al. (1990), drug tests were conducted on 5000 injured drivers in Italy. The proportion of injured drivers with positive drug metabolites was compared with a group of 500 drivers not involved in crashes. The injured drivers had higher proportions of cannabis and stimulants than the comparison group but no statistical tests or actual numbers were presented. Drummer (1995) examined the blood samples of driver fatalities in Australia. Those with stimulants had odds ratios of 1.4 which was not significant. Interestingly, those testing positive for cannabis were less likely than those without drugs to be judged to be responsible for crashes (odds ratio = 0.6). A study by Longo et al. (2000b) utilized drug tests of 2500 injured drivers and culpability analyses that involved an

Fig. 1. The range (i.e. high and low estimates) and means of studies on the prevalence of cannabis metabolites in driver injuries, intentional injuries and other injuries. Note: Means calculated from the total number of subjects in all studies of each category. Number of studies (n) and total number of subjects (N) for each category are as follows: Drivers fatal n = 17, N = 14 668; Drivers nonfatal n = 10, N = 4843; Intentional fatal n = 1, N = 130; Intentional nonfatal n = 3, N = 783; General fatal n = 3, N = 728; General nonfatal n = 8, N = 2597.

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Fig. 2. The range (i.e. high and low estimates) and means of studies on the prevalence of cocaine metabolites in driver injuries, intentional injuries and other injuries. Note: Means calculated from the total number of subjects in all studies of each category. Number of studies (n) and total number of subjects (N) for each category are as follows: Drivers fatal n = 11, N = 9407; Drivers nonfatal n = 7, N = 2652; Intentional fatal n = 5, N=3695; Intentional nonfatal n = 2, N = 721; General fatal n = 3, N = 18 303; General nonfatal n = 8, N = 2469.

objective scoring criterion and analyses of blood samples that determined the concentration of each drug present for assessing culpability of the crashes. Curiously, like the Drummer study, they found that a lower percentage of drivers who tested positive for THC were culpable than drug-free drivers, though the difference was not statistically significant. Similarly, those testing positive for some form of stimulant were not statistically more likely to be culpable for their crashes than drug free drivers; however, only 16 people tested positive for stimulants. Several review articles were published in the past 15 years on the epidemiological research evidence on the role of cannabis in collisions (Bates and Blakely, 1999; Chesher, 1995; Christopherson and Morland, 1997; Crowley and Courtney, 1999; de Gier, 2000; Ferrara et al., 1994; Hunter et al., 1998; Morland, 2000; O’Kane et al., 2002; Robbe and O’Hanlon, 1993; Vingilis and Macdonald, 2002). The majority of literature reviews on cannabis and driving have argued that there is not sufficient scientific evidence to conclude that cannabis use is a risk factor for crashes but better studies are needed. Conclusions by Bates and Blakely (1999) are typical of most reviews on this subject. They concluded that although there is no clear evidence that consumption of cannabis increases the risk of traffic fatalities or injuries, cannabis cannot be excluded as a risk factor for traffic crashes. Two reviewers were stronger in their interpretation of the research evidence. Morland (2000) and O’Kane et al. (2002) suggested that cannabis use does constitute a safety risk for crashes. In arriving at these conclusions, they placed greater emphasis on the pharmacological effects of

cannabis than on epidemiological studies. A smaller subset of these aforementioned review articles also examined the relationship between cocaine use and collisions. These reviews concluded that research has not clearly demonstrated an increased risk of collisions with consumption of cocaine. The failure of studies to obtain significance may be due to methodological limitations, which are described in more detail in the discussion section of this paper. 3.2. Intentional injuries Studies of intentional injuries have focused on homicide and assault victims (see Part B of Table 1). Only one study was found that tested for cannabis metabolites among those who died from intentional injuries. In this study, 33.8% of the homicide victims tested positive for cannabis (Garriott et al., 1986). Since only one study was available, there is insufficient data to draw any meaningful conclusions. Prevalence rates for those with non-fatal intentional injuries who tested positive for cannabis ranged from 4 to 34.2% (M = 10.1%) (see Fig. 1). Studies on the prevalence of cocaine among homicide victims were more common. The proportion of those testing positive for cocaine among those fatally injured ranged from 9 to 40% (M = 30.9%) (Fig. 2). Two studies examined cocaine use among those suffering from non-fatal, intentional injuries. Peden et al. (2000) found that 2.7% of patients with intentional injuries tested positive for cocaine whereas Zautche et al. (1998) found that 20.9% of those with penetrating injuries tested

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positive for cocaine. One study examined reasons for injuries and found that, among those who tested positive for cocaine, 61% died violently (e.g. stabbings or shootings) (Budd et al., 1989). Whether in studies of homicide victims or less serious assaults, the percent of people testing positive for cocaine is generally much higher for intentional injuries than that found in studies of injured drivers. Studies on non-fatal intentional injuries revealed similar prevalence rates of cannabis as studies of injured drivers. 3.3. Injuries in general Several studies have taken drug tests of patients with any type of injury (see Part C of Table 1). The contexts of these injuries are highly heterogeneous which may help to explain the large variation in study results in terms of those who test positive (Macdonald et al., 1999). Figs. 1 and 2 show the range of those who tested positive for either cannabis or cocaine. The proportion of those fatally injured who tested positive for cannabis ranged from 3.9 to 12.0% (M = 11.0%) (see Fig. 1). Those with non-fatal injuries who tested positive for cannabis ranged from 6.3 to 37.0% (M = 15.0%). For cocaine, between 8.0 and 27.4% tested positive with fatal injuries (M = 26.7%) and between 0.0 and 54.0% tested positive with non-fatal injuries (M = 22.2%) (see Fig. 2). These percentages are similar to the ones found for injured drivers for cannabis, but are much higher than the percentages of injured drivers testing positive for cocaine.

4. Survey studies of non-clinical samples Surveys that have investigated the relationship between self-reported drug use and injuries of individuals with no clinical diagnosis of drug abuse are reported in Table 2. 4.1. Injured drivers Surveys in non-clinical samples have been administered to injured drivers during their hospital stay. A few studies examined the frequency of cannabis use and driving. Walsh and Mann (1999) found that 1.9% of Ontario adults reported that they have driven after using cannabis at least once in the previous 12 months. Elliott (1987) found that 43% of young cannabis smokers reported driving while high. These studies reveal that there are cannabis users who drive after using. However, linkages between injury risk and cannabis use could not be made because control groups were not incorporated into those studies. Studies that have investigated the influence of cannabis on collisions have been rare. In a Spanish study, those who reported driving under the influence of drugs were significantly more likely to have collisions than others (del Rió and Alvarez, 1995). Similarly, in New Zealand, Fergusson and Horwood (2001) found that adults aged 18–21 who reported

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using cannabis 50 times per year were significantly more likely to have traffic accidents that resulted from their own driving behaviors than were those who reported no cannabis use (P < 0.001). No surveys in non-clinical samples were found on cocaine and driving. 4.2. Intentional injuries Survey studies and special group studies to determine whether violence is related to cannabis use and cocaine use are more common than studies of drivers. Survey studies investigating the relation between cannabis use and intentional injuries have yielded mixed results. A study involving African–American, inner city young adults revealed that certain criminal activities were related to drug use (Friedman et al., 2001). The degree of cannabis use was positively related to drug trafficking, weapons offences and attempted homicide. Similarly, a national survey of high school students in the US found that those who used alcohol, cannabis or cocaine were more likely to carry weapons and were more likely to be involved in physical fights than those who abstained (Dukarm et al., 1996). In a recent study, cannabis use was found to be highly correlated with violence among adolescents (Harrison et al., 2001). As well, in interviews conducted with 268 individuals convicted of homicide, 4% of the sample indicated that the homicide was related to their marijuana use (Spunt et al., 1994). However, a population survey found that there was no association between self-reported violent injuries and cannabis use (Macdonald and Wells, 2001). In general, survey studies reveal a strong relation between cocaine use and intentional injuries. A study of the streetslevel crack trade in Toronto revealed that the majority of those involved in crack selling transactions had been violently injured (Erickson et al., 2000). In a study of 171 cocaine using women, 62% reported suffering a physical attack (Falck et al., 2001). A similar study of male and female cocaine users who were not in treatment found that 63% had suffered a physical attack (Siegal et al. 2000). Macdonald and Wells (2001) found that those with self-reported violent injuries were significantly more likely than those without injuries to report cocaine use (P < 0.01). Another study found a strong association between violence victimization and self-reported illicit drug use (Odds Ratio = 4.94, P < 0.0001) (Conway et al., 1994). Similarly, an adult general population survey in the US found a strong association between drug use, particularly cocaine use, and being charged with a violent crime (Harrison and Gfroerer, 1992). Harrison et al. (2001) found that cocaine use among adolescents was highly correlated with violence. In addition, abuse of illicit drugs has been found to predict subsequent violent behavior (Friedman, 1998). Consistent statistical links between cocaine and/or cannabis with violence have been observed in these studies. Strong relationships between intentional injuries and crack and free base consumption and/or its sale have been found.

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Table 2 Studies of self-reported drug use and injuries in non-clinical samples Authors

Jurisdiction

Research objective

A. Studies on collisions or injured drivers del Ri´o and Spain Examine incidence of drug Alvarez, 1995 use among drivers

Fergusson and Horwood, 2001

New Zealand

B. Studies on intentional injuries Dukarm et al., USA 1996

Erickson et al., 2002 Friedman et al., 2001

Canada

Harrison et al., 2001

Canada

Macdonald and Wells, 2001

Canada

Spunt et al., 1994

USA

Tardiff et al., 1997

USA

USA

C. Studies on injuries in general Braun et al., 1998 USA

Study group

Findings

1500 drivers

3% had driven and used drugs in the last year, and had been involved in accidents more often than did those who had not driven under the influence (P < 0.001) Those who used cannabis 50 times per year were 1.6 times more likely to have at fault traffic accidents than non-users

Study linkages between cannabis use and traffic accidents

907 individuals aged 21

Study the relation between cocaine and cannabis, and violent behavior

12 272 adolescents

Study violence associated with the crack market Study the relation between illicit drug use and violent criminal offenses

17 male crack sellers

Study the relation between cannabis, cocaine use with violence Identify the factors that are related to violent and accidental injuries

932 students

Study the rates of cannabis use and violent crime Identify profiles for those who are at greatest risk of violence toward others

268 individuals incarcerated for homicide 763 psychiatric patients

Study the relation between self-reported cannabis use and injury incidence Identify the risk factors of traffic, work-related, and other injuries Study the violence experienced by street sex trade workers

2977 current or former cannabis users and 1485 controls 6704 individuals with at least one injury and 6704 controls with no injuries. 30 crack using female prostitutes

612 African–American individuals

10 385 randomly chosen individuals

Chipman, 1995

Canada

Erickson et al., 2000

Canada

Macdonald and Wells, 2001

Canada

Identify factors related to accidental injuries

10 385 randomly chosen individuals

Polen, 1993

USA

Regidor et al., 1996

Spain

Examine the prevalence of injury and illnesses among daily cannabis users Study the factors associated with injuries among cocaine users and the general population

452 “daily” cannabis users and 450 non-smoking controls 369 cocaine users and 4261 controls

Reports of weapon possession among cocaine users (71%) and cannabis users (48%) was significantly higher than for non-users (P < 0.001) Half of the sample had been injured in the course of their crack dealing Cannabis use was positively related with weapons offenses (P < 0.01) and attempted homicide (P < 0.01). Cocaine use was positively related to gang fighting (P < 0.01) and homicide (P < 0.01) Cannabis and cocaine were related to violent behavior Violent injury group was more likely than the accidental injury, and non-injury groups to have used illicit drugs such as cocaine and cannabis. Violent injury group was more likely than the non-injury group to report that the incident was related to alcohol or drug use 7% reported homicide as related to cannabis use. 33% smoked cannabis 24 h before crime Female patients who committed expressive violence were three times more likely to use cocaine than were non-violent patients (P < 0.001)

No statistically significant associations between cannabis use and injury incidence Positive relation between lifetime cannabis and cocaine use and injury (Odds Ratio was 1.26 for cannabis and 1.40 for cocaine). 76% had been injured, most reporting multiple incidents, and 45% had hurt someone else as a result of their involvement in the street crack market No significant differences in the reports of cocaine and cannabis use among those with accidental injuries and those with no injuries Cannabis users showed a small but statistically significant increased risk of respiratory problems, injury, and other types of illnesses Prevalence of injuries was 7.9% among controls and 10.8% among cocaine users

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The studies cannot disentangle whether a similar strong relationship exists between cocaine powder and intentional injuries. There is a pressing need for research that examines the reasons for this link. Some authors have postulated

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that factors such as prior criminality and “social-systemic” drug market involvement might explain these relationships (Denison et al., 1997; Erickson, 2001). However, it has been argued that violence is also determined by various

Table 3 Studies of self-reported drug use and injuries in clinical samples Authors

Jurisdiction

Research objective

A. Studies on collisions or injured drivers Albery et al., 1999 England Examine collision rates among out-of-treatment drug users

Study group

Findings

210 drug users

62.1% of cannabis users drove at least once after using the drug; frequency of driving after using drugs was not significantly related to collisions Those who used cocaine, cannabis, and cocaine and those who used cannabis only had significantly more collisions than matched controls Significant reduction in the average collision rate was found for for alcohol and cocaine subjects compared with controls 50% of the collisions that occurred in the past 5 years occurred under the influence or alcohol and/or drugs There were significant declines in collisions (P < 0.05), drinking-driving convictions (P < 0.001), and moving violations rates (P < 0.001) after treatment Patients had an overall accident rate 1.9 times larger than the expected rates

Chipman et al., 2001

Canada

Compare drug groups in terms of rate of collisions

7 drug abuse groups and polydrug groups

Macdonald et al., 2002

Canada

Mann et al., 1993

Canada

Study effects of treatment on rates of collisions among drug users Examine the contribution of drug use to accident rates

520 alcohol, cannabis and cocaine clients in treatment 144 male substance users aged 21–40

Mann et al., 1995

Canada

Evaluate the effects of substance abuse treatment on accident rates

137 males in treatment for substance use

Smart and Schmidt, 1969

Canada

Investigate accident rates of polydrug abusers excluding alcohol

30 psychiatric patients

Study factors related expressed violence that occurred prior to entering substance abuse treatment Identify the risk profiles related to non-fatal violence victimization Investigate the incidence of violence and post-traumatic stress disorder among cocaine users Determine the prevalence and incidence rates of rape and violence suffered by crack-cocaine users Investigate the correlates of violent injuries among low SES, African–American women Study factors associated with violence suffered by crack-cocaine users Examine various types of violence among individuals in substance abuse treatment

250 males and females in treatment

Over 32% reported they had an incident of physical violence within 90 days prior to treatment

1041 patients

Drug use was clearly associated with violence victimization (odds ratio 3.68)

91 cocaine users seeking treatment

85.7% were physically assaulted at least once

171 female crack-cocaine users, not in treatment

Since using crack-cocaine, 62% reported suffering a physical attack and 32% reported being raped

407 intentionally injured women, 520 women with other health problems 440 crack-cocaine users, not in treatment

Partners of intentionally injured women were more likely to use cocaine than were partners of control group (odds ratio 4.4) Physical attack is widespread among crack-cocaine users

180 males and females in treatment

Greater expressed that violence was related to greater cocaine use and cocaine

Study the association between use of drugs and medical problems upon hospital admission

4526 patients admitted to inpatient substance use service

Cannabis and alcohol users were more likely than other drug users to be hospitalized for trauma related injuries (P < 0.001)

B. Studies on intentional injuries Chermack and Blow, 2002 USA

Conway et al., 1994

USA

Dansky et al., 1999

USA

Falck et al., 2001

USA

Grisso et al., 1999

USA

Siegal et al., 2000

USA

Walton et al., 2002

USA

C. Studies on injuries in general Weintraub et al., 2001 USA

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interrelated factors such as emotional states, motivation, pharmacological influences, and the financial demands of the drug habit (Goldstein, 1985). Other possible reasons for these relationships are described in Section 6. 4.3. Injuries in general There are few studies examining the relationship between other types of injuries and cocaine and cannabis use (see Part C of Table 2). A significant relationship between cannabis use and injuries was found by Polen (1993). However, in a study of 4462 subjects over 3 years, injuries were not related to either current or former use of cannabis (Braun et al., 1998). Similarly, a study by Macdonald and Wells (2001) found that those who experienced an accidental injury were no more likely to use cannabis or cocaine than were those who did not have any injuries. One study revealed that cocaine users had significantly more injuries than non-users (Regidor et al., 1996).

5. Studies using clinical samples of substance abusers in treatment 5.1. Injured drivers One study found that clinical substance abuse populations are likely to drive after using cannabis. For example, in a study of cannabis users in treatment, 62% reported driving at least once after using the drug (Albery et al., 1999). A similar study was not found for driving after using cocaine. Few studies exist that examine collision risks experienced by clinical samples of individuals receiving treatment for cannabis or cocaine abuse (see Part A of Table 3). In the first of these studies, Smart and Schmidt (1969) observed elevated collision rates in abusers of one or more drugs other than alcohol, but the sample was very small (n = 30). In another study of 144 male substance abusers aged 21–40, collision rates in the year before treatment entry were compared with collision rates in the general male population of the same age (Mann et al., 1993). Abusers of stimulants (primarily cocaine) had collision rates about two to three times higher than what would be expected in the general population. The subjects estimated that about 50% of their collisions in the preceding year occurred while they were under the influence of alcohol and/or drugs. As well, results suggested that the frequency of any substance use, as opposed to the use of specific substances, predicted collision involvement and significant post-treatment reductions were found in moving violations, DWI convictions, and total collisions (Mann et al., 1995). A recent study examined the driving risks experienced by large samples of cannabis and cocaine abusers. This study utilized blind linkage procedures to avoid non-respondent bias, and compared the clinical sample to a randomly

selected, frequency-matched (age, gender, location) control group of drivers. Significant elevations in collision risk prior to treatment entry for abusers of both cocaine and cannabis (Chipman et al., 2001) and significant reductions of collisions after treatment were observed (Macdonald et al., 2002). 5.2. Intentional injuries Only three studies were found that focussed on violence among clinical samples of drug abusers and all of them found links to cannabis or cocaine (see Part B of Table 3). Dansky et al. (1999) discovered that 86% of cocaine users seeking treatment had been assaulted at least once. A significant relationship between violence victimization and illicit drug use was found in another study (Conway et al., 1994). The third by Grisso et al. (1999) found that partners of intentionally injured women were more likely to use cocaine than partners of women with other problems (Odds Ratio = 4.4). A recent study of both men and women in treatment found that 32% indicated violence in the 90 days prior to treatment and that both alcohol and cocaine use on the day of the incident were related to violence severity (Chermack and Blow, 2002). Another recent study of treatment subjects indicated that violence expressed towards others was related to greater cocaine use and craving (Walton et al., 2002). 5.3. Injuries in general Only one study was found on injuries in general among cannabis treatment clients and no studies were found for cocaine. Weintraub et al. (2001) found cannabis clients had significantly (P < 0.001) more trauma injuries than other drug clients.

6. Discussion This paper is an important extension of previous reviews for several major reasons. First, the number of studies included in this review makes it the most comprehensive review to date. Second, while previous reviews have tended to focus on collisions, we have also considered violent injuries and injuries in general. Third, it is the first review published where the average percent testing positive has been calculated in all studies combined. Comparison across groups permits an examination of the possible etiologic importance for different types and severity of injuries. In this discussion the strengths and limitations of each of the group of studies are described and conclusions are drawn. In order to better assess whether cannabis or cocaine might be causally related to injuries, it is useful to examine the consistency of results from different types of studies. For cannabis, the preponderance of evidence from laboratory studies show that being under the influence reduces

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psychomotor performance. However, the epidemiological studies using drug testing technology have not clearly shown cannabis to be related to any kind of injury risk. Studies of non-clinical and clinical groups indicate a correlation between cannabis users and injuries; but alternative explanations for the relationships cannot be ruled out. In short the research evidence from the different types of studies is inconsistent. For cocaine, laboratory studies have not shown that ingestion is related to poor psychomotor performance. Epidemiological studies using drug testing technology show an elevated prevalence of cocaine positives among those with intentional injuries compared with other injury groups, especially drivers. Survey studies of both clinical and non-clinical samples also point to a link between cocaine use and violence. Survey research evidence also suggests an association between use and collisions or injuries in general. 6.1. Studies using drug tests Numerous epidemiological studies have been found where drug tests were conducted of those injured. There were no large differences in the proportion testing positive for cannabis for different types of injuries (see Fig. 1). Overall, the highest proportion of people who tested positive for cocaine were those with intentional injuries, followed by injuries in general, while those injured in collisions appear least likely to test positive (see Fig. 2). For crashes, a larger percent of drivers tested positive for cannabis than cocaine, which is not surprising because cannabis is more commonly used than cocaine (Adlaf et al., 1994). Moreover, cannabis has a much longer half-life than cocaine and may be detected for several days after its acute psychoactive effects have ceased (Kapur, 1994). Urinalysis tests are not useful for establishing close temporal proximity between drug use and injuries. Since it is unclear whether people were under the influence of drugs at the time of their injury, drawing a causal connection is problematic. A comparison between injury cases and non-injury controls through analytic epidemiological studies should permit an assessment of whether drug use is a risk factor. The few analytic epidemiological studies that used drug tests and controls have failed to find evidence that cannabis use or cocaine use are related to increased injury risk from collisions. The analytic epidemiological studies have poor statistical power because the presence of drug metabolites is relatively rare and large sample sizes are required to detect significant effects. Moreover, few studies have taken into account the possibility that different drugs might be used in combination with each other, possibly resulting in increased risk for injury. Drug metabolites, for example, are often found in combination with alcohol. Recently, authors have suggested that alcohol in combination with cocaine may increase violent behaviors (Pennings et al., 2002). Although, it is important to separate out the relative role

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of other drugs from alcohol, these studies have not been highly useful in differentiating between the two. The rates of cannabis and cocaine positives have varied substantially from study to study and thus, individual studies cannot be deemed conclusive. The variation in findings among studies is likely due to several factors: jurisdictional differences, random error, different methods of data collection, heterogeneity of samples and different cut-off points for defining drug presence (Bates and Blakely, 1999). The samples likely reflect different environmental and geographical contexts in which injuries occur. However, the differences in the average percent that tested positive among all studies for different injury contexts may be useful for understanding the possible causes and the extent of the problems. The collective knowledge garnered from the studies using drug testing technology do provide useful insights into the injury risks that cannabis and cocaine use pose when different categories of injuries are compared. We might expect that if cannabis or cocaine use was causally related to injuries, the percents testing positive would be higher for fatal than non-fatal injuries. For alcohol, fatally injured drivers are much more likely to be impaired than those with non-fatal injuries. The average percent that tested positive for fatal was lower than non-fatal collisions for cannabis. For fatal collisions, 7.8% tested positive for cannabis and for non-fatal collisions, 11.9% tested positive. The average percent that tested positive for fatal and non-fatal collisions did not differ substantially for cocaine, since 4.6% tested positive in fatal collisions and 4.3% tested positive in non-fatal collisions. The small differences between the average percentage testing positive for fatal versus non-fatal injuries is inconsistent with observations found in the alcohol literature and may point away from these drugs being major causal agents for injuries due to collisions. With regard to intentional injuries, the average percent testing positive in fatal and non-fatal injuries differed substantially; however, conclusions cannot be drawn for cannabis because only one study was found on fatal homicides. In the one study on cannabis, 33.8% of homicide victims tested positive (Garriott et al., 1986) whereas the average proportion who tested positive for cannabis among those non-fatally injured was 10.1%. Similarly, the average percentage testing positive for cocaine among homicide victims was 30.9% while the average percentage of two studies for those non-fatally injured victims revealed that 17.4% tested positive for cocaine. It should also be noted that the drug prevalence rates across the studies on intentional injuries varied greatly. For those who were non-fatally injured, some studies reported on those who suffered penetrating injuries (e.g. Zautche et al., 1998), whereas other studies investigated all types of violent injuries. The average percent testing positive in fatal versus non-fatal injuries in general was not very different for either cannabis or cocaine. For injuries in general, a slightly higher proportion of fatal injuries involved cocaine than

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non-fatal injuries. In addition, a lower proportion of fatal injuries involved cannabis than non-fatal injuries. For injury contexts, a comparison of interest is between the high percent that tested positive for cocaine for intentional injuries compared with all other groups, but especially the driver groups. On average, the percentage of people with intentional injuries who tested positive for cocaine was almost seven times higher than the percentage among cocaine—positive, injured (fatally and non-fatally) drivers. The percentages that were cocaine positive for injuries in general were 26.7% for fatalities and 22.2% for non-fatal injuries. While these latter rates are also large, the samples from most of these studies included those injured intentionally, which explain these high levels. The high percent of intentional injuries that tested positive for cocaine points toward cocaine as a possible causal agent in violence. This conclusion should be made cautiously because other factors might better explain these relationships. Also, the samples were not randomly selected (practically all the samples were homicide victims in the United States) and might be biased for example, if researchers decided to conduct studies in areas where cocaine is known to be a problem. The percent that tested positive for cannabis with fatal and non-fatal intentional injuries was only 13.5%, which was about the same percent for other types of injuries. The low rate of cannabis use and intentional injuries compared with the very high rate of cocaine provides further support that the two drugs have different etiological relationships with violence. These observations are consistent with other research that has found a link between aggression and cocaine but not with cannabis. 6.2. Studies of non-clinical samples For collisions, no studies were found on cocaine and driver injuries and only two studies were found on cannabis. The two studies on cannabis and injured drivers found a statistical link. In terms of injuries in general, results of studies are mixed and no definitive conclusions can be drawn. Survey studies indicated that both cannabis and cocaine use are related to violence. However, few studies were found on violent behavior and drug use and existing studies have focused on special populations like inner-city or incarcerated individuals. Although links have been noted between cannabis or cocaine use and violence, much more research is required to demonstrate a causal connection. A major limitation of practically all studies using survey methods is that associations between drug use and injuries may be spurious. Other factors related to both drug use and injuries may influence injury risk. These factors are the set or attitudes of the person (including physical condition and personality structure), the pharmacological action of the drug itself, and the setting or influence of the social and physical surroundings in which the use occurs (Zinberg, 1984). Recent studies and reviews on set variables, such as aggression

(Beirness, 1993; Deffenbacher et al., 2000; Gidron et al., 2001; Wiesenthal et al., 2000), risk-taking/impulsiveness (Beirness, 1993; Jonah, 1997; Vavrik, 1997), stress (Norris et al., 2000; Simon and Corbett, 1996; Veneziano and Veneziano, 1992), fatigue (Connor et al., 2001; Horstmann et al., 2000; Masa et al., 2000), and criminality (Denison et al., 1997; Wells-Parker et al., 1986) confirm the importance of these characteristics in predicting collisions and injury risk. Studies have found that many of the characteristics described above are over-represented in substance abuse populations, which might also explain higher injury rates. Setting variables such as peer group and physical context of drug use may also increase injury risk. Several variables related to drug use are potentially related to injury risk. Frequency of use, severity of substance abuse problems, concurrent use of other drugs or alcohol, and individual reactions to drugs should be investigated. Withdrawal effects from cocaine and cannabis, such as exhaustion, anxiety, agitation, mood swings and depression and long term effects of abuse, such as chronic sleep disruption, distractibility, irritability and depression (Coambs and McAndrews, 1994; Cohen and Sas, 1993; Herscovitch, 1996) could also increase injury risks. Another disadvantage of survey studies of non-clinical populations is that self-reports can be biased toward under-reporting of use, especially in surveys of general populations. As well, those that admit drug use may also be more likely to admit to violence and other injuries. However, the depth of knowledge that can be gained by survey approaches cannot be achieved by any other method. Interestingly, most survey studies have focussed on whether a relationship exists between drug use and injuries, which was the same focus of studies using drug tests. Few studies have addressed the temporal proximity of drug use with injuries, and few studies have attempted to rule out other factors that might explain significant relationships between drug use and injuries. 6.3. Studies using clinical samples The majority of survey studies of clinical and general populations have found a significant relationship between cannabis or cocaine use and different types of injuries. The studies have found both cannabis and cocaine are related to collisions and violence; however, few studies have been conducted, making conclusions tentative. These studies have not provided a good indication of whether the relationships found are causal or correlational. The relationship between clinical samples of cannabis and cocaine abusers with injuries in general is not clear. One of the strengths of studies on clinical samples is the accessibility and validity of information gathered. Good validity of self-reports has been established among substance users both during and after treatment (Hindin et al., 1994; Nelson et al., 1998). Response rates of individuals in

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treatment is quite high and it is possible to attain additional information that has already been collected as part of the treatment process. Although these studies suffer from the same limitations as survey studies of non-clinical samples, the biases related to self-reports of drug use are not as pronounced in the clinical samples. Poor agreement between drug tests and self-reports of drug use have been found from the general population and juvenile arrests (Fendrich and Xu, 1994; Fendrich et al., 1999). It is argued that those who are seeking treatment have already acknowledged that they have a problem and as a result, are more likely to provide accurate accounts regarding that problem. Overall, the only consistent relationship found across the three classes of studies was between cocaine and violence. Studies of clinical samples, although very few in number, were the most likely to find relationships with injuries. 6.4. Directions for future research The current review is an important synthesis of the literature regarding drug use and injury risk. The prevalence rates of cannabis and cocaine use for various injury types vary, depending on factors such as how the sample was collected, the method of measuring drug impairment, the presence of drug dependence, and the jurisdiction of the study. Despite this variation in methodology, it was revealed that, among hospital patient samples, the highest prevalence rate of cocaine use was for intentional injuries. The prevalence rate for cannabis metabolites was quite consistent for different injury contexts and for fatal and non-fatal injuries. This review reveals the need for further research in the area of injury risk and substance use. Future studies that use drug tests can still provide a significant contribution. First, analytic epidemiological studies, with designs similar to the one conducted by Marquet et al. (1998), are useful for understanding relative risks; however, future studies of this kind should include much larger sample sizes and blood tests should be used. One impediment to conducting such studies is that use of drug tests without consent may not be ethically acceptable in some jurisdictions. Some hospitals justify tests without consent if they are considered useful for health-related diagnostic purposes (Stoduto et al., 1993). Another useful approach that is less ethically restrictive was utilized by Longo et al. (2000b) where the proportion responsible for their crash or injury who test positive are compared with the same proportion who are not responsible. This approach is especially feasible for fatalities. Another improvement for future studies that use drug tests is to incorporate self-reported information on drug use and other risk factors for injuries. This approach could be particularly useful for assessing the validity of self-reports; however, if consent is required the utility will be limited. The difficulty associated with measuring the degree of drug impairment has impacted negatively on the accuracy of

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the prevalence rates of drug use. An area for research that has received scant attention is understanding the impact of chronic use of cannabis and cocaine on injury risk. Such research is especially important for cocaine because the acute effects of the drug do not effect performance but evidence presented in this review shows a strong link to violence. In addition, much more research is needed on people who use multiple drugs. Cocaine users for example, frequently use alcohol (Cohen and Sas, 1993). Further understanding of how drugs interact with one another and whether their combination of use produces synergistic effects in terms of injury risk is needed. Future survey research of both the general and clinical populations should continue to address issues of causality between drug use and injuries. Besides demonstrating that significant relationships exist, two requirements of causality need further investigation. The first is demonstrating drug impairment at the time when injuries occur. The second requirement is that the relative contribution of other variables that might also explain injury and collision risk should be evaluated. Studies should include the larger constellation of variables that are known to be related to injury and collision risk. Variables that should be addressed in future research include personality characteristics, risk-taking, aggression, criminality, and stressful life events, in order to control for their potential confounding influence on the relationship between use of cannabis and cocaine and risk of injuries and collisions.

Acknowledgements This paper received partial financial support from the Canadian Institute of Health Research.

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