No real surprises in the meta-analysis described below but it certainly puts another line in the sand on the link between cannabis and mental health. The question remains on whether governments and NGOs can wait for a definitive link before making large policy changes.

Cannabis Use and Earlier Onset of Psychosis: A Systematic Meta-analysis
Matthew Large [et al.] Archives of General Psychiatry, 7th February 2011

Abstract:
Context A number of studies have found that the use of cannabis and other psychoactive substances is associated with an earlier onset of psychotic illness.

Objective To establish the extent to which use of cannabis, alcohol, and other psychoactive substances affects the age at onset of psychosis by meta-analysis.

Data Sources Peer-reviewed publications in English reporting age at onset of psychotic illness in substance-using and non–substance-using groups were located using searches of CINAHL, EMBASE, MEDLINE, PsycINFO, and ISI Web of Science.

Study Selection Studies in English comparing the age at onset of psychosis in cohorts of patients who use substances with age at onset of psychosis in non–substance-using patients. The searches yielded 443 articles, from which 83 studies met the inclusion criteria.

Data Extraction Information on study design, study population, and effect size were extracted independently by 2 of us.

Data Synthesis Meta-analysis found that the 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. Differences in the proportion of cannabis users in the substance-using group made a significant contribution to the heterogeneity in the effect sizes between studies, confirming an association between cannabis use and earlier mean age at onset of psychotic illness.

Conclusions The results of meta-analysis provide evidence for a relationship between cannabis use and earlier onset of psychotic illness, and they support the hypothesis that cannabis use plays a causal role in the development of psychosis in some patients. The results suggest the need for renewed warnings about the potentially harmful effects of cannabis.

Full text available here – http://archpsyc.ama-assn.org/cgi/content/full/archgenpsychiatry.2011.5