Cannabis use increases the risk of psychotic disorders in a dose-dependent manner, while evidence for affective outcomes is weaker and less consistent.
Summary
Systematic review (k=35 longitudinal population studies); pooled adjusted OR for psychotic outcome with ever cannabis use = 1,41 (95% CI 1,20–1,65); dose-response effect: OR=2,09 (95% CI 1,54–2,84) with frequent use. Evidence consistent with an independent causal effect of cannabis on psychosis risk; depression/anxiety outcomes less consistently supported.
P
PopulationGeneral population (longitudinal population studies), pooled sample size not explicitly reported
I
InterventionCannabis use (ever/frequent)
C
ControlNo cannabis use
O
OutcomeIncreased risk of psychotic outcomes with ever use (pooled adjusted OR=1,41, 95% CI 1,20–1,65); with most frequent use OR=2,09 (95% CI 1,54–2,84); less consistent findings for depression, suicidal ideation and anxiety
Confidence in the evidence
Very lowLowModerateHigh
High
The highest of four GRADE levels, the effect estimate is very reliable.
<h4>Background</h4>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.<h4>Methods</h4>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 included, 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.<h4>Findings</h4>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.<h4>Interpretation</h4>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.