Effects of cannabis use on outcomes of psychotic disorders: systematic review

Background It is unclear if research findings support clinical opinion that cannabis use leads to worse outcomes in people with psychosis, or whether this impression is confounded by other factors. Aims To systematically review the evidence pertaining to whether cannabis affects outcome of psychotic disorders. Method We searched 10 relevant databases (to November 2006) reference lists of included studies and contacted experts. We included 13 longitudinal studies from 15303 references. Data extraction and quality assessment were conducted independently and in duplicate. Results Cannabis use was consistently associated with increased relapse and non-adherence. Associations with other outcome measures were more disparate. Few studies adjusted for baseline illness severity, and most made no adjustment for alcohol, or other potentially important confounders. Adjusting for even a few confounders often resulted in substantial attenuation of results. Conclusions Confidence that most associations reported were specifically due to cannabis is low. Despite clinical opinion, it remains important to establish whether cannabis is harmful, what outcomes are particularly susceptible, and how such effects are mediated. Studies to examine this further are eminently feasible. Declaration of interest P.B.J. and T.R.E.B. were both invited experts on the Advisory Council on the Misuse of Drugs Cannabis Review in 2005. A.L.-H. has received an honorarium from Sanofi-Aventis for attending a meeting about cannabinoid antagonists. S.Z., P.B.J., T.R.E.B., G.L. and A.L.-H. have all received honoraria for lectures and talks, or consultancy fees (for work unrelated to cannabis) from pharmaceutical companies.