This week’s issue of The Lancet contains the most comprehensive meta-analysis to date of the link between cannabis and psychosis:
The evidence is consistent with the view that cannabis increases risk of psychotic outcomes independently of confounding and transient intoxication effects, although evidence for affective outcomes is less strong. The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies such as those reviewed here. However, we conclude that there is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life.
The journal’s podcast includes an interview with Stanley Zimmet, of the Department of Psychological Medicine at Cardiff University, who is one of the authors of the study.
I’m inclined to think that the long-term effects of cannabis (and those of any other psychotropic drug) very much depend on the mental composition of the individual using the drug. Therefore, smoking cannabis may increase the risk of psychosis or affective disorders, if one is somehow already predisposed to such a condition. But finding a correlation between two factors does not mean that one causes the other.