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Age of first use, current marijuana use and driving after use among Canadian high school students
Addictive Behaviors 90 (2019) 329–333
Contents lists available at ScienceDirect
Addictive Behaviors journal homepage: www.elsevier.com/locate/addictbeh
Short Communication
Age of first use, current marijuana use and driving after use among Canadian high school students
T
Sunday Azagbaa, , Mark Asbridgeb ⁎
a b
Department of Family & Preventive Medicine, University of Utah, United States Department of Community Health and Epidemiology, Dalhousie University, Nova Scotia, Canada
HIGHLIGHTS
age at first marijuana use was associated with current marijuana use • Younger found stronger associations for early age at first use, especially before age 15. • We • Age at first marijuana use was associated with driving after use ARTICLE INFO
ABSTRACT
Keywords: Marijuana Youth Frequency of consumption Age of first use of marijuana Driving under the influence
Early exposure to marijuana may have lasting effects into adulthood given that adolescence is a critical period during which most substance use is initiated. Recreational marijuana recently became legal in Canada, with a legal age of purchase set as either 18 or 19 years, depending on the province. Historically, the mean age of onset for marijuana use among youth occurs at a much younger age. The current study examined the association between age at first use of marijuana with current marijuana use patterns and driving after use among high school students. Data on 24,630 high school students (grades 9–12) were from the 2014–2015 Canadian Student Tobacco, Alcohol and Drugs Survey. We restricted our analyses to the students who reported having ever used marijuana and had data on age at first marijuana use (n = 6709). Multivariable logistic and negative binomial regressions were performed to examine the association between current marijuana use patterns and age at first marijuana use. Students with younger age at first use of marijuana had significantly higher odds of current marijuana use, a higher frequency of consumption, and were more likely to drive after use of marijuana. Effective enforcement of age restriction laws in combination with other prevention efforts may be beneficial in delaying the age of onset of marijuana until late adolescence and early adulthood.
1. Introduction Outside of alcohol, marijuana is the most widely used psychoactive substance in North America, particularly among youth and young adults (World Health Organization, n.d.; Azofeifa, 2016; Health Canada, 2017a, 2017b; Johnston et al., 2018; United Nations Office on Drugs and Crime, 2016). While there are potential therapeutic benefits from marijuana, its use, especially frequent, long-term use, has been associated with various adverse effects (Hall and Degenhardt, 2009; Volkow et al., 2014). There has been an upward trend in the potency of marijuana in the last three decades (ElSohly et al., 1984, 2000, 2016). Frequent and long-term consumption has been linked to the development of paranoia and psychosis, and to a lesser extent, cardiovascular disease and dependence, while acute consumption leads to impaired
⁎
respiratory function and affects psychomotor performance, heightening the risk of motor vehicle crashes and injury (Asbridge et al., 2012; Elvik, 2013; Hall and Degenhardt, 2009; Kalant, 2004; Li et al., 2011; Rogeberg and Elvik, 2016; Volkow et al., 2014). In addition, prior studies have shown that early marijuana onset is associated with juvenile offending, school dropout, unemployment and welfare dependence (Arria et al., 2015; Fergusson et al., 2015; Fergusson and Horwood, 1997; McCaffrey et al., 2010). Despite these harms, the perceived risk associated with the use of marijuana has continued to decline. Data from the Monitoring the Future national survey showed that the proportion of students who perceived marijuana use to be harmful has decreased significantly in the last two decades among US students in the 8th, 10th, and 12th grades (Johnston et al., 2018). The change in public attitude may have
Corresponding author at: Division of Public Health, Department of Family & Preventive Medicine, University of Utah, Salt Lake City, UT 84108, United States. E-mail address: [email protected] (S. Azagba).
https://doi.org/10.1016/j.addbeh.2018.11.042 Received 5 September 2018; Received in revised form 26 November 2018; Accepted 26 November 2018 Available online 28 November 2018 0306-4603/ © 2018 Elsevier Ltd. All rights reserved.
Addictive Behaviors 90 (2019) 329–333
S. Azagba, M. Asbridge
contributed to a policy environment that is rapidly evolving from strict to permissive in many countries (Duff et al., 2012; Health Canada, 2017a, 2017b; NIAAA's APIS, 2017; von Hoffmann, 2016; Williamson, 2017). In Canada, recreational use of marijuana recently became legal (October 17, 2018), and some states in the United States have implemented similar laws, with more forthcoming (Health Canada, 2017a, 2017b; NIAAA's APIS, 2017). One area of considerable debate is deciding the most appropriate age at which youth can purchase and consume marijuana. While the federal legalization has set the minimum legal access age to purchase recreational marijuana at 18, provinces can increase the access age, with some opting for a minimal legal age of 19, like the minimum legal drinking age (MLDA). Conversely, governing health bodies, including the Canadian Medical Association (CMA) and the Canadian Psychiatric Association, proposed a minimum legal age of 21. Of particular concern is that early and frequent exposure to marijuana may have longerlasting effects into adulthood given that adolescence is a critical period during which most substance use is initiated (DeWit et al., 2000; Ehlers et al., 2007; Fergusson and Horwood, 1997; Kandel and Raveis, 1989). Although many young people engage in marijuana use – in Canada, 21% of older adolescents (aged 15–19) and 30% of youth aged 20–24 years report marijuana use in the previous year (Leos-Toro et al., 2017) – delaying age of onset into early adulthood has been shown to reduce short- and longer-term harms. Recent research on the impact of MLDA laws demonstrate their impact on reducing excess consumption and alcohol-related harms (Callaghan et al., 2016a; Callaghan et al., 2016b; Carpenter and Dobkin, 2011; Dejong and Blanchette, 2014 & Plunk et al., 2016). In the present study, we examined the association between age of first marijuana use, current marijuana use and driving after use of marijuana. This study is particularly timely given that the findings may have potential implications for provincial regulatory approaches around the enforcement of minimal legal purchase age. In addition, few national population-based studies in the extant literature have examined age of cannabis initiation and subsequent or current use patterns (Dewit et al., 2000; Ehlers et al., 2007; Kandel and Raveis, 1989). Some of the earlier findings may not reflect the rapidly changing norms around marijuana. Not much is known about the association between age at first marijuana use and driving under the influence of marijuana from a large population-based sample (Le Strat et al., 2015). Using a nationally representative sample of Canadian high school students, this study examined the association between age at first marijuana use, and current use behavior and driving after use of marijuana.
requirements, parents provided permission for their child to participate in the study via active parent permission or active information-passive permission protocols. Only students with parental permission were invited to participate in the study on the day of survey implementation. Students were not remunerated and could opt out at any time. A total of 336 schools (47% of those approached) and 42,094 students (66% response rate) in grades 6 to 12 participated in the 2014/2015 cycle. We restricted the sample for the present study to those in high school (grades 9–12). 2.2. Measures 2.2.1. Dependent variables Marijuana use was measured from the question on how often participants used marijuana or cannabis in the last 30 days, with the following response options: did not use marijuana in the last 30 days; once or twice; once or twice weekly; 3 or 4 times a week; 5 or 6 times a week; and every day. Three separate dichotomous variables and one count variable were used to represent levels of marijuana use pattern in the 30 days prior to the survey. The dichotomous variables created included (i) current use – students who reported using marijuana at least once in the last 30 days; (ii) at least once weekly – students who reported using marijuana at least once weekly in the last 30 days and (iii) everyday – students who reported using marijuana every day in the last 30 days. For the count outcome variable (total frequency of marijuana use), we interpolated how often marijuana is used by taking the midpoint of the responses bins (did not use marijuana in the last 30 days; once or twice; once or twice weekly; 3 or 4 times a week; 5 or 6 times a week; and every day). Specifically, a value ‘0’ was assigned to those who did not use marijuana in the last 30 days; ‘1.5’ for once or twice; ‘6.5’ for once or twice a week (midpoint of the bin, 1.5 was multiplied by the average number of weeks, 4.35, in a month to derive the monthly total); ‘15.2’ for 3 or 4 times a week (midpoint of the bin, 3.5, was multiplied by the average number of weeks); ‘23.9’ for 3 or 4 times a week (midpoint of the bin, 3.5, was multiplied by the average number of weeks) and ‘30’ for everyday marijuana use. Driving after use of marijuana was measured by respondents' responses to whether they had driven a vehicle within 2 h of using marijuana. Driving after use represents students who reported driving a vehicle within 2 h of using marijuana in the last 30 days. 2.2.2. Independent variables The main independent variable of interest, age at first marijuana use, was derived among those who ever used marijuana from the question: “How old were you when you first used marijuana or cannabis?” “Response options were: “I do not know”, “8 years or younger”, “9 years”, “10 years”, “11 years” “12 years”, “13 years”, “14 years”, “15 years”, “16 years”, “17 years”, and “18 years or older”. The following categories of age at first marijuana use were created: ≤ 12 years, 13 years, 14 years, 15 years, 16 years, and ≥ 17 years. We combined age at first use at 12 years or younger (≤ 12 years), and 17 or 18 years (≥17 years) due to sample size. The “I do not know” responses were considered missing observations.
2. Methods 2.1. Data The present study used data from the 2014–2015 Canadian Student Tobacco, Alcohol and Drugs Survey (CSTADS). The CSTADS (formally known as the Youth Smoking Survey) is a cross-sectional, school-based survey of a generalizable sample of students in grades 6 through 12. The survey tracks adolescent and youth substance use behavior. The target population for CSTADS consists of Canadians attending private, public, and Catholic schools enrolled in grades 6 to 12 (6 to secondary V in Quebec). The survey excluded those living on First Nations reserves, Canada's three northern Territories (Yukon, Nunavut, and Northwest Territories), and those attending special schools or schools on military bases. CSTADS uses a stratified single-stage cluster design, with strata based on health region smoking rate and type of school. In each province, two or three health region smoking rate strata and two schoollevel strata were defined. Random selection of schools within each stratum ensured a generalizable sample of schools for each province. Research ethics boards at multiple levels approved the study: Health Canada, the University of Waterloo, and institutions and school boards in each participating province. Consistent with school board
2.3. Statistical analysis The analysis included 6709 students in grades 9–12, of whom 52.1% were male. The percentage of students in each grade for our sample, included 12.7% for grade 9, 21.6% (grade 10), 31.1% (grade 11), and 34.6% (grade 12). Multivariable logistic regressions were performed for the dichotomous outcomes to examine the association between age at marijuana onset, current marijuana use patterns, and driving under the influence of marijuana. The analyses adjusted for a number of variables, including gender, grade level, ever cigarette- and alcohol-use status, level of urbanization (urban, rural), school-level median household income – measured continuously (the first three digits of the postal code 330
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higher frequency of marijuana use by factors of 5.12, 3.25, and 2.01, respectively. Adjacent contrasts between age at first use showed statistically significant differences (up to age 14 versus age 15), with early age at first use associated with more frequent use of marijuana. Results also showed that marijuana use at younger age use was significantly associated with driving after use of marijuana. Fig. 1 shows the predicted outcome variables occurring based on age at first use, with probabilities shown for dichotomous variables and counts for frequency of marijuana use. High school students who used marijuana at early age had higher probability of current marijuana use and driving after use. Equally, they were likely to use marijuana more frequently (Fig. 1).
of the respondent's school was used to extract school-level median household income from the 2011 Canadian Census data), and region of residence—East (Newfoundland and Labrador, Prince Edward Island, New Brunswick, and Nova Scotia), West (Saskatchewan, Alberta, Manitoba, and British Columbia), Quebec, and Ontario. Adjusted odds ratios (AOR) with the 95% confidence interval (CI) from the multivariable logistic regression are reported. For frequency of marijuana use, we used a negative binomial model in order to account for its count nature and correct for potential overdispersion of our data. Incidence rate ratios (IRR) with the 95% confidence interval (CI) from the negative binomial regression are reported. Survey weights were used in all analyses to produce population estimates and to adjust for unequal probabilities of selection. Regression analyses adjusted for potential serial correlation by clustering standard errors at the school level. All analyses were conducted using Stata 15.0 (Stata Corp, College Station, Texas).
4. Discussion This brief report examined the association between age at marijuana onset and current use behavior using a representative sample of Canadian high school students. The findings suggest early age at first use, especially before age 15, was significantly associated with current marijuana use and more frequent marijuana consumption and aligns with the previous body of research on early age of marijuana onset (Dewit et al., 2000; Ehlers et al., 2007; Fergusson and Horwood, 1997; Kandel and Raveis, 1989). In keeping with a prior study, we found that younger age at first use of marijuana was significantly associated with driving after use of marijuana (Le Strat et al., 2015). Using data from the 2001–2002 National Epidemiological Survey on Alcohol and Related Conditions, Le Strat and colleagues found that those who used cannabis before age 14 were 3 times more likely to reported having driven under the influence of cannabis. It is unclear how Canadian adult recreational marijuana legalization will impact youth. However, given that many Canadian youth consume marijuana prior to age 18, with an average age at first use of 14.3, it will be important to examine whether the establishment of a minimum legal age of purchase as part of a regulated legal market will produce similar reductions on marijuana consumption patterns and related harms that are observed with respect to the MLDA. Considerable research will be required to assess the impacts of legalization on youth consumption in Canada, its impacts on age of onset, use over the life course, and the potential health and social consequences. The findings of this study should be considered along with some of its limitations. First, this study makes no claim on the causal relationship between age at first use of marijuana and subsequent marijuana use patterns. Second, the use of self-reported data including age at marijuana onset may be subject to recall and reporting bias. Third, information about having a driver's license is not available. Finally, while the study controlled for selected individual characteristics, it is possible that there are other important factors not accounted for in the analysis (e.g. sensation seeking, parent's education, household socioeconomic status). Notwithstanding these limitations, using a large and nationally representative data of high school students, the current study showed that younger age at marijuana onset is significantly associated with current marijuana use and driving under the influence of marijuana.
3. Results The weighted descriptive statistics of marijuana use (n = 6709) and driving after use of marijuana (n = 6647) by age at first use is reported in Table 1. Among high school students who reported having ever used marijuana in their lifetime, the weighted results showed that about 53% used marijuana at least once in the last 30 days, and 27% used marijuana at least once weekly in the last 30 days. About 10% of students in grades 9 to 12 drove a vehicle within 2 h of using marijuana in the last 30 days. The average age at first marijuana use was 14.3 years. Current marijuana use and driving after use of marijuana were higher for those with younger age at first use (Table 1). Table 2 reports the results from the multivariable logistic regression showing a statistically significant association between the age at first use and current marijuana use patterns. Age at first marijuana use was significantly associated with current use of marijuana when compared to first use at age 17 or older (≤ 12 years, AOR = 4.01, CI = 2.08–7.74; 13 years, AOR = 2.34, CI = 1.25–4.37; 14 years, AOR = 2.11, CI = 1.15–3.86). Equally, a younger age at first use was significantly associated with more frequent use of marijuana (at least weekly or everyday). Table 2 also reports adjusted odds ratios from adjacent contrasts of age at first marijuana use. For example, in column 3, those that had their first marijuana use at age 13 were more likely to use marijuana at least once weekly in the last 30 days (AOR = 1.70, CI = 1.21–2.38) compared to first use at age 14. Likewise, marijuana use at age 14 was significantly associated with higher odds of current marijuana use at least weekly (AOR = 1.77, CI = 1.22–2.58) relative to those who had their first use at age 15. For frequency of marijuana use in the last 30 days, the results from negative binomial regression showed that early age at first use was significantly associated with higher frequency of marijuana use. Specifically, those that had their first marijuana use at age 12 compared to first use at age 17 and older, were expected to have higher frequency of marijuana use by a factor of 7.95 (IRR = 7.95, CI = 4.62–13.67). Similarly, compared to first use at age 17, marijuana use at ages 13, 14 and 15 were expected to have
Table 1 Descriptive statistics of marijuana use (n = 6709) and driving after use of marijuana (n = 6647) by age at first use.
Age at first marijuana use ≤ 12 years (n = 959) 13 years (n = 1049) 14 years (n = 1732) 15 years (n = 1614) 16 years (n = 1015) ≥ 17 years (n = 340) Aggregate
Current use weighted %
At least once weekly weighted %
Daily use weighted %
Frequency of use weighted count
Driving after use weighted %
75.5 61.3 57.2 47.1 40.3 38.3 53.1
60.5 38.7 27.9 17.0 15.2 9.4 26.7
34.6 13.6 7.1 3.6 3.7 3.2 9.5
14.8 7.8 5.0 3.2 2.6 1.9 5.4
26.5 13.6 7.3 7.5 7.0 7.1 10.4
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Table 2 Multivariable regression of marijuana use in the past month on age at first use (n = 6709).
Age at first marijuana use ≤12 years 13 years 14 years 15 years 16 years ≥ 17 years
Current use AOR (CI)
At least once weekly AOR (CI)
Daily use AOR (CI)
Frequency of use IRR (CI)
Driving after use AOR (CI)
4.01 (2.08–7.74) 2.34 (1.25–4.37) 2.11 (1.15–3.86) 1.41 (0.79–2.53) 1.03 (0.57–1.86) Ref
13.21 (5.87–29.72) 6.66 (3.00–14.78) 3.92 (1.76–8.72) 2.21 (1.00–4.87) 1.77 (0.78–4.02) Ref
14.61 (3.69–57.92) 5.57 (1.40–22.13) 2.71 (0.69–10.63) 1.40 (0.35–5.58) 1.20 (0.25–5.72) Ref
7.95 (4.62–13.67) 5.12 (2.96–8.88) 3.27 (1.92–5.57) 2.01 (1.19–3.41) 1.65 (0.90–3.02) Ref
5.78 (2.07–16.14) 3.38 (1.18–9.64) 1.50 (0.54–4.20) 1.67 (0.58–4.77) 1.18 (0.39–3.59) Ref
1.98 (1.37–2.87) 1.70 (1.21–2.38) 1.77 (1.22–2.58) 1.25 (0.76–2.05)
2.62 (1.69–4.08) 2.05 (1.23–3.42) 1.93 (1.03–3.62 1.17 (0.43–3.18)
1.55 (1.24–1.95) 1.57 (1.22–2.01) 1.62 (1.26–2.09) 1.22 (0.84–1.76)
1.71 (1.08–2.71) 2.25 (1.40–3.62) 0.90 (0.53–1.54) 1.41 (0.71–2.81)
Adjacent contrast of age at first marijuana use ≤ 12 vs 13 years 1.72 (1.14–2.60) 13 vs 14 years 1.11 (0.80–1.53) 14 vs 15 years 1.50 (1.13–1.98) 15 vs 16 years 1.38 (0.99–1.91)
Ref = reference category. AOR = adjusted odds ratios obtained from multivariable logistics regressions. IRR = incidence rate rations obtained from negative binomial regression. CI = 95% confidence interval. Analyses adjusted for sex, grade level, socioeconomic status, cigarette use, alcohol use, living area, and region of residence.
Fig. 1. Predictive margins of current marijuana use and driving after use of marijuana by age at first marijuana use.
Funding source
the final manuscript as submitted. Mark Asbridge: Dr. Asbridge contributed to the drafting of the manuscript, and critically reviewed the manuscript. MA approved the final manuscript as submitted.
None. Contributors
Conflict of interest
Sunday Azagba: Dr. Azagba conceptualized and designed the study, analyzed the data, and drafted and revised the manuscript. SA approved
None. 332
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S. Azagba, M. Asbridge
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Contents lists available at ScienceDirect
Addictive Behaviors journal homepage: www.elsevier.com/locate/addictbeh
Short Communication
Age of first use, current marijuana use and driving after use among Canadian high school students
T
Sunday Azagbaa, , Mark Asbridgeb ⁎
a b
Department of Family & Preventive Medicine, University of Utah, United States Department of Community Health and Epidemiology, Dalhousie University, Nova Scotia, Canada
HIGHLIGHTS
age at first marijuana use was associated with current marijuana use • Younger found stronger associations for early age at first use, especially before age 15. • We • Age at first marijuana use was associated with driving after use ARTICLE INFO
ABSTRACT
Keywords: Marijuana Youth Frequency of consumption Age of first use of marijuana Driving under the influence
Early exposure to marijuana may have lasting effects into adulthood given that adolescence is a critical period during which most substance use is initiated. Recreational marijuana recently became legal in Canada, with a legal age of purchase set as either 18 or 19 years, depending on the province. Historically, the mean age of onset for marijuana use among youth occurs at a much younger age. The current study examined the association between age at first use of marijuana with current marijuana use patterns and driving after use among high school students. Data on 24,630 high school students (grades 9–12) were from the 2014–2015 Canadian Student Tobacco, Alcohol and Drugs Survey. We restricted our analyses to the students who reported having ever used marijuana and had data on age at first marijuana use (n = 6709). Multivariable logistic and negative binomial regressions were performed to examine the association between current marijuana use patterns and age at first marijuana use. Students with younger age at first use of marijuana had significantly higher odds of current marijuana use, a higher frequency of consumption, and were more likely to drive after use of marijuana. Effective enforcement of age restriction laws in combination with other prevention efforts may be beneficial in delaying the age of onset of marijuana until late adolescence and early adulthood.
1. Introduction Outside of alcohol, marijuana is the most widely used psychoactive substance in North America, particularly among youth and young adults (World Health Organization, n.d.; Azofeifa, 2016; Health Canada, 2017a, 2017b; Johnston et al., 2018; United Nations Office on Drugs and Crime, 2016). While there are potential therapeutic benefits from marijuana, its use, especially frequent, long-term use, has been associated with various adverse effects (Hall and Degenhardt, 2009; Volkow et al., 2014). There has been an upward trend in the potency of marijuana in the last three decades (ElSohly et al., 1984, 2000, 2016). Frequent and long-term consumption has been linked to the development of paranoia and psychosis, and to a lesser extent, cardiovascular disease and dependence, while acute consumption leads to impaired
⁎
respiratory function and affects psychomotor performance, heightening the risk of motor vehicle crashes and injury (Asbridge et al., 2012; Elvik, 2013; Hall and Degenhardt, 2009; Kalant, 2004; Li et al., 2011; Rogeberg and Elvik, 2016; Volkow et al., 2014). In addition, prior studies have shown that early marijuana onset is associated with juvenile offending, school dropout, unemployment and welfare dependence (Arria et al., 2015; Fergusson et al., 2015; Fergusson and Horwood, 1997; McCaffrey et al., 2010). Despite these harms, the perceived risk associated with the use of marijuana has continued to decline. Data from the Monitoring the Future national survey showed that the proportion of students who perceived marijuana use to be harmful has decreased significantly in the last two decades among US students in the 8th, 10th, and 12th grades (Johnston et al., 2018). The change in public attitude may have
Corresponding author at: Division of Public Health, Department of Family & Preventive Medicine, University of Utah, Salt Lake City, UT 84108, United States. E-mail address: [email protected] (S. Azagba).
https://doi.org/10.1016/j.addbeh.2018.11.042 Received 5 September 2018; Received in revised form 26 November 2018; Accepted 26 November 2018 Available online 28 November 2018 0306-4603/ © 2018 Elsevier Ltd. All rights reserved.
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contributed to a policy environment that is rapidly evolving from strict to permissive in many countries (Duff et al., 2012; Health Canada, 2017a, 2017b; NIAAA's APIS, 2017; von Hoffmann, 2016; Williamson, 2017). In Canada, recreational use of marijuana recently became legal (October 17, 2018), and some states in the United States have implemented similar laws, with more forthcoming (Health Canada, 2017a, 2017b; NIAAA's APIS, 2017). One area of considerable debate is deciding the most appropriate age at which youth can purchase and consume marijuana. While the federal legalization has set the minimum legal access age to purchase recreational marijuana at 18, provinces can increase the access age, with some opting for a minimal legal age of 19, like the minimum legal drinking age (MLDA). Conversely, governing health bodies, including the Canadian Medical Association (CMA) and the Canadian Psychiatric Association, proposed a minimum legal age of 21. Of particular concern is that early and frequent exposure to marijuana may have longerlasting effects into adulthood given that adolescence is a critical period during which most substance use is initiated (DeWit et al., 2000; Ehlers et al., 2007; Fergusson and Horwood, 1997; Kandel and Raveis, 1989). Although many young people engage in marijuana use – in Canada, 21% of older adolescents (aged 15–19) and 30% of youth aged 20–24 years report marijuana use in the previous year (Leos-Toro et al., 2017) – delaying age of onset into early adulthood has been shown to reduce short- and longer-term harms. Recent research on the impact of MLDA laws demonstrate their impact on reducing excess consumption and alcohol-related harms (Callaghan et al., 2016a; Callaghan et al., 2016b; Carpenter and Dobkin, 2011; Dejong and Blanchette, 2014 & Plunk et al., 2016). In the present study, we examined the association between age of first marijuana use, current marijuana use and driving after use of marijuana. This study is particularly timely given that the findings may have potential implications for provincial regulatory approaches around the enforcement of minimal legal purchase age. In addition, few national population-based studies in the extant literature have examined age of cannabis initiation and subsequent or current use patterns (Dewit et al., 2000; Ehlers et al., 2007; Kandel and Raveis, 1989). Some of the earlier findings may not reflect the rapidly changing norms around marijuana. Not much is known about the association between age at first marijuana use and driving under the influence of marijuana from a large population-based sample (Le Strat et al., 2015). Using a nationally representative sample of Canadian high school students, this study examined the association between age at first marijuana use, and current use behavior and driving after use of marijuana.
requirements, parents provided permission for their child to participate in the study via active parent permission or active information-passive permission protocols. Only students with parental permission were invited to participate in the study on the day of survey implementation. Students were not remunerated and could opt out at any time. A total of 336 schools (47% of those approached) and 42,094 students (66% response rate) in grades 6 to 12 participated in the 2014/2015 cycle. We restricted the sample for the present study to those in high school (grades 9–12). 2.2. Measures 2.2.1. Dependent variables Marijuana use was measured from the question on how often participants used marijuana or cannabis in the last 30 days, with the following response options: did not use marijuana in the last 30 days; once or twice; once or twice weekly; 3 or 4 times a week; 5 or 6 times a week; and every day. Three separate dichotomous variables and one count variable were used to represent levels of marijuana use pattern in the 30 days prior to the survey. The dichotomous variables created included (i) current use – students who reported using marijuana at least once in the last 30 days; (ii) at least once weekly – students who reported using marijuana at least once weekly in the last 30 days and (iii) everyday – students who reported using marijuana every day in the last 30 days. For the count outcome variable (total frequency of marijuana use), we interpolated how often marijuana is used by taking the midpoint of the responses bins (did not use marijuana in the last 30 days; once or twice; once or twice weekly; 3 or 4 times a week; 5 or 6 times a week; and every day). Specifically, a value ‘0’ was assigned to those who did not use marijuana in the last 30 days; ‘1.5’ for once or twice; ‘6.5’ for once or twice a week (midpoint of the bin, 1.5 was multiplied by the average number of weeks, 4.35, in a month to derive the monthly total); ‘15.2’ for 3 or 4 times a week (midpoint of the bin, 3.5, was multiplied by the average number of weeks); ‘23.9’ for 3 or 4 times a week (midpoint of the bin, 3.5, was multiplied by the average number of weeks) and ‘30’ for everyday marijuana use. Driving after use of marijuana was measured by respondents' responses to whether they had driven a vehicle within 2 h of using marijuana. Driving after use represents students who reported driving a vehicle within 2 h of using marijuana in the last 30 days. 2.2.2. Independent variables The main independent variable of interest, age at first marijuana use, was derived among those who ever used marijuana from the question: “How old were you when you first used marijuana or cannabis?” “Response options were: “I do not know”, “8 years or younger”, “9 years”, “10 years”, “11 years” “12 years”, “13 years”, “14 years”, “15 years”, “16 years”, “17 years”, and “18 years or older”. The following categories of age at first marijuana use were created: ≤ 12 years, 13 years, 14 years, 15 years, 16 years, and ≥ 17 years. We combined age at first use at 12 years or younger (≤ 12 years), and 17 or 18 years (≥17 years) due to sample size. The “I do not know” responses were considered missing observations.
2. Methods 2.1. Data The present study used data from the 2014–2015 Canadian Student Tobacco, Alcohol and Drugs Survey (CSTADS). The CSTADS (formally known as the Youth Smoking Survey) is a cross-sectional, school-based survey of a generalizable sample of students in grades 6 through 12. The survey tracks adolescent and youth substance use behavior. The target population for CSTADS consists of Canadians attending private, public, and Catholic schools enrolled in grades 6 to 12 (6 to secondary V in Quebec). The survey excluded those living on First Nations reserves, Canada's three northern Territories (Yukon, Nunavut, and Northwest Territories), and those attending special schools or schools on military bases. CSTADS uses a stratified single-stage cluster design, with strata based on health region smoking rate and type of school. In each province, two or three health region smoking rate strata and two schoollevel strata were defined. Random selection of schools within each stratum ensured a generalizable sample of schools for each province. Research ethics boards at multiple levels approved the study: Health Canada, the University of Waterloo, and institutions and school boards in each participating province. Consistent with school board
2.3. Statistical analysis The analysis included 6709 students in grades 9–12, of whom 52.1% were male. The percentage of students in each grade for our sample, included 12.7% for grade 9, 21.6% (grade 10), 31.1% (grade 11), and 34.6% (grade 12). Multivariable logistic regressions were performed for the dichotomous outcomes to examine the association between age at marijuana onset, current marijuana use patterns, and driving under the influence of marijuana. The analyses adjusted for a number of variables, including gender, grade level, ever cigarette- and alcohol-use status, level of urbanization (urban, rural), school-level median household income – measured continuously (the first three digits of the postal code 330
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higher frequency of marijuana use by factors of 5.12, 3.25, and 2.01, respectively. Adjacent contrasts between age at first use showed statistically significant differences (up to age 14 versus age 15), with early age at first use associated with more frequent use of marijuana. Results also showed that marijuana use at younger age use was significantly associated with driving after use of marijuana. Fig. 1 shows the predicted outcome variables occurring based on age at first use, with probabilities shown for dichotomous variables and counts for frequency of marijuana use. High school students who used marijuana at early age had higher probability of current marijuana use and driving after use. Equally, they were likely to use marijuana more frequently (Fig. 1).
of the respondent's school was used to extract school-level median household income from the 2011 Canadian Census data), and region of residence—East (Newfoundland and Labrador, Prince Edward Island, New Brunswick, and Nova Scotia), West (Saskatchewan, Alberta, Manitoba, and British Columbia), Quebec, and Ontario. Adjusted odds ratios (AOR) with the 95% confidence interval (CI) from the multivariable logistic regression are reported. For frequency of marijuana use, we used a negative binomial model in order to account for its count nature and correct for potential overdispersion of our data. Incidence rate ratios (IRR) with the 95% confidence interval (CI) from the negative binomial regression are reported. Survey weights were used in all analyses to produce population estimates and to adjust for unequal probabilities of selection. Regression analyses adjusted for potential serial correlation by clustering standard errors at the school level. All analyses were conducted using Stata 15.0 (Stata Corp, College Station, Texas).
4. Discussion This brief report examined the association between age at marijuana onset and current use behavior using a representative sample of Canadian high school students. The findings suggest early age at first use, especially before age 15, was significantly associated with current marijuana use and more frequent marijuana consumption and aligns with the previous body of research on early age of marijuana onset (Dewit et al., 2000; Ehlers et al., 2007; Fergusson and Horwood, 1997; Kandel and Raveis, 1989). In keeping with a prior study, we found that younger age at first use of marijuana was significantly associated with driving after use of marijuana (Le Strat et al., 2015). Using data from the 2001–2002 National Epidemiological Survey on Alcohol and Related Conditions, Le Strat and colleagues found that those who used cannabis before age 14 were 3 times more likely to reported having driven under the influence of cannabis. It is unclear how Canadian adult recreational marijuana legalization will impact youth. However, given that many Canadian youth consume marijuana prior to age 18, with an average age at first use of 14.3, it will be important to examine whether the establishment of a minimum legal age of purchase as part of a regulated legal market will produce similar reductions on marijuana consumption patterns and related harms that are observed with respect to the MLDA. Considerable research will be required to assess the impacts of legalization on youth consumption in Canada, its impacts on age of onset, use over the life course, and the potential health and social consequences. The findings of this study should be considered along with some of its limitations. First, this study makes no claim on the causal relationship between age at first use of marijuana and subsequent marijuana use patterns. Second, the use of self-reported data including age at marijuana onset may be subject to recall and reporting bias. Third, information about having a driver's license is not available. Finally, while the study controlled for selected individual characteristics, it is possible that there are other important factors not accounted for in the analysis (e.g. sensation seeking, parent's education, household socioeconomic status). Notwithstanding these limitations, using a large and nationally representative data of high school students, the current study showed that younger age at marijuana onset is significantly associated with current marijuana use and driving under the influence of marijuana.
3. Results The weighted descriptive statistics of marijuana use (n = 6709) and driving after use of marijuana (n = 6647) by age at first use is reported in Table 1. Among high school students who reported having ever used marijuana in their lifetime, the weighted results showed that about 53% used marijuana at least once in the last 30 days, and 27% used marijuana at least once weekly in the last 30 days. About 10% of students in grades 9 to 12 drove a vehicle within 2 h of using marijuana in the last 30 days. The average age at first marijuana use was 14.3 years. Current marijuana use and driving after use of marijuana were higher for those with younger age at first use (Table 1). Table 2 reports the results from the multivariable logistic regression showing a statistically significant association between the age at first use and current marijuana use patterns. Age at first marijuana use was significantly associated with current use of marijuana when compared to first use at age 17 or older (≤ 12 years, AOR = 4.01, CI = 2.08–7.74; 13 years, AOR = 2.34, CI = 1.25–4.37; 14 years, AOR = 2.11, CI = 1.15–3.86). Equally, a younger age at first use was significantly associated with more frequent use of marijuana (at least weekly or everyday). Table 2 also reports adjusted odds ratios from adjacent contrasts of age at first marijuana use. For example, in column 3, those that had their first marijuana use at age 13 were more likely to use marijuana at least once weekly in the last 30 days (AOR = 1.70, CI = 1.21–2.38) compared to first use at age 14. Likewise, marijuana use at age 14 was significantly associated with higher odds of current marijuana use at least weekly (AOR = 1.77, CI = 1.22–2.58) relative to those who had their first use at age 15. For frequency of marijuana use in the last 30 days, the results from negative binomial regression showed that early age at first use was significantly associated with higher frequency of marijuana use. Specifically, those that had their first marijuana use at age 12 compared to first use at age 17 and older, were expected to have higher frequency of marijuana use by a factor of 7.95 (IRR = 7.95, CI = 4.62–13.67). Similarly, compared to first use at age 17, marijuana use at ages 13, 14 and 15 were expected to have
Table 1 Descriptive statistics of marijuana use (n = 6709) and driving after use of marijuana (n = 6647) by age at first use.
Age at first marijuana use ≤ 12 years (n = 959) 13 years (n = 1049) 14 years (n = 1732) 15 years (n = 1614) 16 years (n = 1015) ≥ 17 years (n = 340) Aggregate
Current use weighted %
At least once weekly weighted %
Daily use weighted %
Frequency of use weighted count
Driving after use weighted %
75.5 61.3 57.2 47.1 40.3 38.3 53.1
60.5 38.7 27.9 17.0 15.2 9.4 26.7
34.6 13.6 7.1 3.6 3.7 3.2 9.5
14.8 7.8 5.0 3.2 2.6 1.9 5.4
26.5 13.6 7.3 7.5 7.0 7.1 10.4
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Table 2 Multivariable regression of marijuana use in the past month on age at first use (n = 6709).
Age at first marijuana use ≤12 years 13 years 14 years 15 years 16 years ≥ 17 years
Current use AOR (CI)
At least once weekly AOR (CI)
Daily use AOR (CI)
Frequency of use IRR (CI)
Driving after use AOR (CI)
4.01 (2.08–7.74) 2.34 (1.25–4.37) 2.11 (1.15–3.86) 1.41 (0.79–2.53) 1.03 (0.57–1.86) Ref
13.21 (5.87–29.72) 6.66 (3.00–14.78) 3.92 (1.76–8.72) 2.21 (1.00–4.87) 1.77 (0.78–4.02) Ref
14.61 (3.69–57.92) 5.57 (1.40–22.13) 2.71 (0.69–10.63) 1.40 (0.35–5.58) 1.20 (0.25–5.72) Ref
7.95 (4.62–13.67) 5.12 (2.96–8.88) 3.27 (1.92–5.57) 2.01 (1.19–3.41) 1.65 (0.90–3.02) Ref
5.78 (2.07–16.14) 3.38 (1.18–9.64) 1.50 (0.54–4.20) 1.67 (0.58–4.77) 1.18 (0.39–3.59) Ref
1.98 (1.37–2.87) 1.70 (1.21–2.38) 1.77 (1.22–2.58) 1.25 (0.76–2.05)
2.62 (1.69–4.08) 2.05 (1.23–3.42) 1.93 (1.03–3.62 1.17 (0.43–3.18)
1.55 (1.24–1.95) 1.57 (1.22–2.01) 1.62 (1.26–2.09) 1.22 (0.84–1.76)
1.71 (1.08–2.71) 2.25 (1.40–3.62) 0.90 (0.53–1.54) 1.41 (0.71–2.81)
Adjacent contrast of age at first marijuana use ≤ 12 vs 13 years 1.72 (1.14–2.60) 13 vs 14 years 1.11 (0.80–1.53) 14 vs 15 years 1.50 (1.13–1.98) 15 vs 16 years 1.38 (0.99–1.91)
Ref = reference category. AOR = adjusted odds ratios obtained from multivariable logistics regressions. IRR = incidence rate rations obtained from negative binomial regression. CI = 95% confidence interval. Analyses adjusted for sex, grade level, socioeconomic status, cigarette use, alcohol use, living area, and region of residence.
Fig. 1. Predictive margins of current marijuana use and driving after use of marijuana by age at first marijuana use.
Funding source
the final manuscript as submitted. Mark Asbridge: Dr. Asbridge contributed to the drafting of the manuscript, and critically reviewed the manuscript. MA approved the final manuscript as submitted.
None. Contributors
Conflict of interest
Sunday Azagba: Dr. Azagba conceptualized and designed the study, analyzed the data, and drafted and revised the manuscript. SA approved
None. 332
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