The British Journal of Psychiatry (2008) 193: 357-363. doi: 10.1192/bjp.bp.107.046375
© 2008 The Royal College of Psychiatrists
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REVIEW ARTICLE

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

Stanley Zammit, PhD

Academic Unit of Psychiatry, University of Bristol, and Department of Psychological Medicine, Cardiff University

Theresa H. M. Moore, MSc, BSc and Anne Lingford-Hughes, BM, BCh, PhD

Academic Unit of Psychiatry, University of Bristol

Thomas R. E. Barnes, MD, DSc

Department of Psychological Medicine, Imperial College London

Peter B. Jones, MD, PhD

Department of Psychiatry, Cambridge University

Margaret Burke, BA, MSc

Department of Social Medicine, University of Bristol

Glyn Lewis, PhD

Academic Unit of Psychiatry, University of Bristol, UK

Correspondence: Stanley Zammit, Department of Psychiatry, Cardiff University, Heath Park, Cardiff CF14 4XN, UK. Email: zammits{at}cardiff.ac.uk

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.

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 15 303 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.


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