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Review: Cannabis use increases the risk of psychotic outcomes

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Stanley Zammit

Correspondence to: Dr Stanley Zammit, Department of Psychological Medicine, Cardiff University, Cardiff CF14 4XN, UK;



Does cannabis use increase the risk of psychotic or affective mental health problems?


Psychosis and affective mental health problems



Systematic review of longitudinal studies.

Data sources:

MEDLINE, EMBASE, CINAHL, PsycINFO, ISI Web of Knowledge, ISI Proceedings, ZETOC, BIOSIS, LILACS and MEDCARIB; searched from database inception to September 2006.

Study selection and analysis:

Population-based longitudinal studies or nested case-control studies assessing cannabis use and subsequent psychotic or affective mental health problems were included. The following were explicitly excluded: cohort studies of people with metal illness or substance-use problems, cohorts in prison populations, randomised controlled trials of cannabis for medical use. The presence of delusions, hallucinations or thought disorder was a requirement for all psychosis outcomes. Affective outcomes included mood or bipolar disorder, affective disorder, depression anxiety, neurosis, mania or suicide ideation and attempts. Studies were pooled using DerSimonian and Laird random effects models.


Seven cohort studies assessing psychosis were included. …

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  • Source of funding: Department of Health, UK.


  • Competing interests: None.