The Lancet 2007; 370:319-328

DOI:10.1016/S0140-6736(07)61162-3

Cannabis use and risk of psychotic or affective mental health
outcomes: a systematic review
Theresa HM Moore MSc a,   Dr Stanley Zammit PhD  a c ,   Anne Lingford-
Hughes PhD a,   Thomas RE Barnes DSc d,   Peter B Jones PhD e,
Margaret Burke MSc b   and   Glyn Lewis PhD a

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 effect, 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 effect was present for both psychotic and affective
outcomes.

Interpretation
The evidence is consistent with the view that cannabis increases risk
of psychotic outcomes independently of confounding and transient
intoxication effects, although evidence for affective 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 sufficient
evidence to warn young people that using cannabis could increase their
risk of developing a psychotic illness later in life.

Affiliations

a. Academic Unit of Psychiatry, University of Bristol, Bristol, UK
b. Department of Social Medicine, University of Bristol, Bristol, UK
c. Department of Psychological Medicine, Cardiff University, Cardiff,
UK
d. Department of Psychological Medicine, Imperial College, London, UK
e. Department of Psychiatry, Cambridge University, Cambridge, UK

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

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This is important -- kids need to know about it.  Unfortunately, I'm
not sure they're going to believe it.

Marilyn