Cannabis use and mental health in young people: cohort study

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Abstract
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. Participants: A statewide 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. What is already known on this topic Frequent recreational use of cannabis has been linked to high rates of depression and anxiety in cross sectional surveys and studies of long term users Why cannabis users have higher rates of depression and anxiety is uncertain Previous longitudinal studies of cannabis use in youth have not analysed associations with frequent cannabis use What this study adds A strong association between daily use of cannabis and depression and anxiety in young women persists after adjustment for intercurrent use of other substances Frequent cannabis use in teenage girls predicts later higher rates of depression and anxiety Depression and anxiety in teenagers do not predict later cannabis use; self medication is therefore unlikely to be the reason for the association