Trajectories of Marijuana Use in Canadian Youth and Associations with Substance Use, Mental Health, and Behavioral Problems in Adolescence and Young Adulthood

Introduction: Compared to other countries, Canadian youth are the highest users of marijuana in the developed world, with 28% of youth under 15 reporting marijuana use in the last 12 months (2013 UNICEF Office of Research report). Using data from cohorts of Canadian youth spanning ages 12 to 28, we address the following questions: Is there a safer age for onset of use? How do the trajectories of use vary across the transition to young adulthood? How do these relate to the use of other substances and behavioral and mental health problems? We estimate variability in marijuana use trajectories using latent class growth curve analyses and we examine the association of patterns of marijuana use with use of tobacco, alcohol and illicit drugs and with mental health and behavioral problems at both the onset of these trajectories (ages 15 to 18) and at the end of these trajectories (ages 24 to 28).

Methods: The Victoria Healthy Youth Survey is a 10-year prospective study of a random community sample of 662 (48% male; M_age = 15.5) youth followed biennially for six assessments from 2003 to 2013. Models were run using Mplus 7.11. 

Results: Five marijuana use groups were identified: Abstainers (29%), Occasional Users (27% who used a few times a month), Decreasers (14% who began use by age 15 and dropped steadily to no use by age 28), Increasers (20% who began use by age 15 and increased to high frequencies of use [i.e. more than once a week] in the early 20’s before starting to decline) and Chronic Users (11% who began with high frequencies of use by age 13.5 and remained at high levels of use at age 28). Lower use groups had later onset of use. Chronic Users had more problem behaviors (ODD, ADHD and conduct problems) in both adolescence and young adulthood and more depressive symptoms in young adulthood than other groups. Increasers were similar to Chronic users, but reported more illicit drug use in adolescence than other groups and lower levels of mental health problems in young adulthood. Decliners reported more depressive symptoms in adolescence than other groups, but less than Chronic Users in young adulthood, and they were less likely to co-use other substances in young adulthood.

Conclusions: The profiles of use suggest that resolution of adolescent levels of mental health, alcohol use, and behavioral problems may be important for limiting chronic marijuana use.