Theresa H M Moore, Stanley Zammit, Anne Lingford-Hughes, Thomas R E Barnes, Peter B Jones, Margaret Burke, Glyn Lewis
Summary
Background – Whether cannabis can cause psychotic or affective symptoms that persist beyond transient intoxication is unclear. We systematically reviewed the evidence pertaining to cannabis use and occurrence of psychotic or affective 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 those reviewed here. However, we conclude that there is now suffi cient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life.

Source: The Lancet Vol.370 pp 319-328 July 2007