The Archives of General Psychiatry has published a study (full text available here) that provides additional evidence for a relationship between cannabis use and earlier onset of psychotic illness.
Conducted by Matthew Large, BSc (Med), MBBS, FRANZCP, Swapnil Sharma, MBBS, FRANZCP, Michael T. Compton, MD, MPH, Tim Slade, PhD, and Olav Nielssen, MBBS, MCrim, FRANZCP, the study supports the hypothesis that cannabis use plays a causal role in the development of psychosis in some patients and suggests the need for renewed warnings about the potentially harmful effects of cannabis.
The results found that the age at onset of psychosis for cannabis users was almost three years younger than for non-users. For those with broadly defined substance use, the age at onset of psychosis was two years younger than for non-users.
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, which researchers believe confirms an association between cannabis use and earlier mean age at onset of psychotic illness.
Alcohol use was not associated with a significantly earlier age at onset of psychosis.