www.thelancet.com Vol 370 July 28, 2007
Cannabis use and risk of psychotic or aff ective mental health
outcomes: a systematic review
Theresa H M Moore, Stanley Zammit, Anne Lingford-Hughes, Thomas R E Barnes, Peter B Jones, Margaret Burke, Glyn Lewis
Background Whether cannabis can cause psychotic or aff ective symptoms that persist beyond transient intoxication is
unclear. We systematically reviewed the evidence pertaining to cannabis use and occurrence of psychotic or aff ective
mental health outcomes.
Methods We searched Medline, Embase, CINAHL, PsycINFO, ISI Web of Knowledge, ISI Proceedings, ZETOC,
BIOSIS, LILACS, and MEDCARIB from their inception to September, 2006, searched reference lists of studies selected
for inclusion, and contacted experts. Studies were included if longitudinal and population based. 35 studies from
4804 references were included. Data extraction and quality assessment were done independently and in duplicate.
Findings There was an increased risk of any psychotic outcome in individuals who had ever used cannabis (pooled
adjusted odds ratio=1·41, 95% CI 1·20–1·65). Findings were consistent with a dose-response eff ect, with greater risk in
people who used cannabis most frequently (2·09, 1·54–2·84). Results of analyses restricted to studies of more clinically
relevant psychotic disorders were similar. Depression, suicidal thoughts, and anxiety outcomes were examined separately.
Findings for these outcomes were less consistent, and fewer attempts were made to address non-causal explanations,
than for psychosis. A substantial confounding eff ect was present for both psychotic and aff ective outcomes.
Interpretation The evidence is consistent with the view that cannabis increases risk of psychotic outcomes
independently of confounding and transient intoxication eff ects, although evidence for aff ective outcomes is less
strong. The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal
studies such as th