This meta-analysis adds more weight to the argument that cannabis use plays a causal role in the development of psychosis in some patients.

The review pooled 83 studies that reported on age at onset of psychotic illness in substance-using and non–substance-using groups.

The study found that:

• Age at onset of psychosis for cannabis users was 2.70 years younger (standardized mean difference = –0.414) than for nonusers
• For those with broadly defined substance use, the age at onset of psychosis was 2.00 years younger (standardized mean difference = –0.315) than for nonusers
• Alcohol use was not associated with a significantly earlier age at onset of psychosis

Large, M. Sharma, S. Compton, MT. Slade, T. Nielssen, O. Cannabis Use and Earlier Onset of Psychosis: A Systematic Meta-analysis. Arch Gen Psychiatry. 2011;68(6):555-561. doi:10.1001/archgenpsychiatry.2011.5 [Abstract]