Marijuana trajectories in Canadian youth: Associations with substance use and mental health

We differentiated marijuana-use trajectories in a large cohort of Canadian youth and compared the use of other substances, mental health symptoms, and behavioural problems for each of the identified trajectories at their baselines in adolescence (ages 12 to 18) and their endpoints (ages 22 to 29). Data came from the Victoria Healthy Youth Survey, a 10-year prospective study of a random community sample of 662 participants in Victoria, British Columbia, Canada (48% male; M_age = 15.5). Canadian youth were followed biennially for six assessments from 2003 to 2013. Five distinct marijuana-use classes were identified using latent-class growth-curve analyses: abstainers (29%), occasional users (27%), decreasers (14%), increasers (20%), and chronic users (11%). Lower use classes typically began use after age 15. Chronic users had more problem behaviours (e.g., attention-deficit/hyperactivity disorder, oppositional- defiant disorder, and conduct problems) in both adolescence and young adulthood and more depressive symptoms in young adulthood than other classes. Decreasers reported more depressive symptoms in adolescence than chronic users and were less likely to co-use other substances in young adulthood. Increasers were similar to chronic users in young adulthood, but reported more illicit drug use and lower levels of depressive and oppositional- defiant disorder symptoms. Problematic marijuana use occurs in the context of mental health and problem behaviours as well as other substance-use concerns. Prevention and treatment approaches need to include anticipation and treatment of co-occurring problems to stem negative effects of marijuana during the transition from adolescence to young adulthood.