title: An overview of systematic reviews on cannabis and psychosis: discussing apparently conflicting results.
abstract: OBJECTIVE
  Cross-sectional surveys have revealed that cannabis is the most widely used illicit substance in Western countries. Cannabis intoxication can lead to acute, transient psychotic symptoms and the short-term exacerbation of pre-existing psychotic symptoms. However, controversy exists about whether cannabis can actually cause long-term psychosis.
  
  
  METHODS
  We summarised the findings of systematic reviews on the association between cannabis use and psychosis, searching MEDLINE, EMBASE and CINAHL up to August 2007. We assessed the methodological quality, selected the better quality reviews and analysed reasons for discordant results.
  
  
  RESULTS
  We included five systematic reviews. Four of the reviews performed a meta-analysis and showed a consistent association between cannabis use and psychosis; the fifth review considered psychological problems more broadly, did not perform a meta-analysis and reported an inconsistent association. The reasons for discordance were: different outcomes (psychosis vs. psychological problems), different inclusion criteria for primary studies and different methods for summarising the results.
  
  
  CONCLUSIONS
  This overview shows a consistent association between cannabis use and psychotic symptoms, though it is not possible to draw firm conclusions about a causal relationship. Reverse causality and residual confounding cannot be excluded. An interaction with other environmental and genetic factors is difficult to ascertain.
  
  
  CONCLUSIONS
  We conclude that there is insufficient knowledge to determine the level of risk associated with cannabis use in relation to psychotic symptoms and that more information is needed on both the risks of cannabis use and the benefits of preventive interventions to support evidence-based approaches in this area.
authors.affiliation: Department of Epidemiology, Local Health Unit Roma E, Roma, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Minozzi
authors.forename: Marina
authors.initials: M
authors.lastname: Davoli
authors.forename: Anna M
authors.initials: AM
authors.lastname: Bargagli
authors.forename: Laura
authors.initials: L
authors.lastname: Amato
authors.forename: Simona
authors.initials: S
authors.lastname: Vecchi
authors.forename: Carlo A
authors.initials: CA
authors.lastname: Perucci
mesh_terms: D006801:Humans; D002189:Marijuana Abuse / Q000150:complications*; D015201:Meta-Analysis as Topic; D011618:Psychotic Disorders / Q000150:complications* / Q000209:etiology*