Purpose: This study aimed to determine if the associations between age at cannabis first use and mental health and depression persist after controlling for the misuse of cannabis, other illicit drugs, alcohol and cigarettes.
Methods: Baseline data from an ongoing cohort study on substance use risk factors were used (N ¼ 5521). The association between age at cannabis first use and the frequency of cannabis use was assessed using a Cox proportional hazard model. Associations with other illicit drug use, alcohol dependence, nicotine dependence, mental health and depression were tested using linear regressions and logistic regressions.
Results: Participants using cannabis ''almost every day'' were on an average two years younger at cannabis first use than those using cannabis ''once a month or less''. Age at cannabis first use was also associated with other illicit drug use, alcohol dependence and nicotine dependence. Associations with mental health and depression were substantially attenuated after adjustment for the misuse of cannabis and other substances.
Conclusions: These findings show that early cannabis onset is associated with later impairments in mental health and depression; however, these are largely explained by later substance abuse. Effective preventive measures are needed to delay cannabis onset and to avoid the progression from cannabis to using other drugs.