The Effect of Age of Initiation of Cannabis Use on Psychosis, Depression, and Anxiety among Youth under 25 Years

Objectives: This study was conducted to review the current state of evidence on the association between age of initiation of cannabis use and symptoms of psychosis, depression, or anxiety among youth under 25 years of age. Methods: We conducted a systematic review of articles published prior to March 2018 by searching OVID MEDLINE, PsycINFO, EMBASE, and the references of included studies. We included comparative studies (cohort, case-control, cross-sectional) that reported on cannabis use in persons <25 years of age (exposure) and symptoms of psychosis, depression, or anxiety (outcome). We narratively synthesized the studies according to design (cohort, etc.) and psychiatric outcome. We used the Newcastle-Ottawa Scale to assess risk of bias. Results: Of the 534 citations identified through the literature search, 23 met the eligibility criteria and were included in this review. With psychosis as the outcome, all except one study found that earlier cannabis use was generally associated with higher risks. With depression/anxiety as the outcome, 6 of the 11 included studies reported findings indicating that earlier use of cannabis was linked to higher symptom levels. Conclusion: In persons <25 years old, greater cannabis use is associated with more psychological symptoms, especially among those with a predisposition or existing vulnerability to such outcomes (Oxford Centre for Evidence-Based Medicine level 3 or 4). Policy makers need to consider the adverse effects of cannabis use in youth when planning a public health approach to cannabis legalization.