Objective: To determine whether cannabis use in adolescence predisposes to higher rates of depression and anxiety in young adulthood.

Design: Seven wave cohort study over six years.

Setting: 44 schools in the Australian state of Victoria
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Participants: A state wide secondary school sample of 1601 students aged 14-15 followed for seven years.

Main outcome measure: Interview measure of depression and anxiety (revised clinical interview schedule) at wave 7.

Results: Some 60% of participants had used cannabis by the age of 20; 7% were daily users at that point. Daily use in young women was associated with an over fivefold increase in the odds of reporting a state of depression and anxiety after adjustment for intercurrent use of other substances (odds ratio 5.6, 95% confidence interval 2.6 to 12). Weekly or more frequent cannabis use in teenagers predicted an approximately twofold increase in risk for later depression and anxiety (1.9, 1.1 to 3.3) after adjustment for potential baseline confounders. In contrast, depression and anxiety in teenagers predicted neither later weekly nor daily cannabis use.
Conclusions: Frequent cannabis use in teenage girls predicts later depression and anxiety, with daily users carrying the highest risk. Given recent increasing levels of cannabis use, measures to reduce frequent and heavy recreational use seem warranted.