Cannabis as a causal factor for psychosis — a review of the evidence

Abstract

Adults suffering from psychosis have high rates of cannabis use. However, there remains controversy as to whether this is a cause or consequence of the psychosis. This review tests the hypothesis that cannabis can cause psychosis by reviewing studies verifying three important criteria for causality in psychiatry: association, temporal priority, and direction, with the emphasis put on prospective longitudinal population-based studies that tested temporal priority. Evidence supports a causal role for cannabis in the development of psychosis but it appears that cannabis use is neither a sufficient nor a necessary cause for psychosis. Rather, it is a component cause - part of a complex constellation of factors leading to psychosis. On an individual level, cannabis use appears to confer only a two-to three-fold increase in the relative risk for later schizophrenia. On a population level, elimination of cannabis use could lead to a 7-13% reduction in incidence of schizophrenia. Thus, some cases of psychotic disorder could be prevented by discouraging cannabis use, particularly among vulnerable youths. More research is needed to better understand the mechanisms by which cannabis causes psychosis.