Initiation and progression of cannabis use in a population-based Australian adolescent longitudinal study

Aims. To examine predictors of cannabis use initiation, continuity and progression to daily use in adolescents. Design. Population-based cohort study over 3 years with 6 waves of data collection. Participants. 2032 students, initially aged 14-15 years, from 44 secondary schools in the state of Victoria, Australia. Measurements. Self-report cannabis use was categorized on four levels (none, any, weekly, daily) and summarized as mid-school (waves 2/3) and late-school (waves 4/5/6) use. Background, school environment, mid-school peer use and individual characteristics were assessed. Findings. Peer cannabis use, daily smoking, alcohol use, antisocial behaviour and high rates of school-level cannabis use were associated with mid-school cannabis use and independently predicted late-school uptake. Cannabis use persisted into late-school use in 80% of all mid-school users. Persisting cannabis use from mid- to late-school was more likely in regular users (odds ratio (OR) 3.4), cigarette smokers (OR any smoking: 2.0, daily smoking: 3. 3) and those reporting peer use (OR 2.1). Mid-school peer use independently predicted incident late-school daily use in males (OR 6.5) while high-dose alcohol use (OR 6.1) and antisocial behaviour (OR 6.6) predicted incident late-school daily use in females. Conclusions. Most cannabis use remained occasional during adolescence but escalation to potentially harmful daily use in the late-school period occurred in 12% of early users. Transition was more likely in males, for whom availability and peer use were determinants. In contrast, females with multiple extreme behaviours were more likely to become daily users. Cigarette smoking was an important predictor of both initiation and persisting cannabis use.