ISSUES: 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. APPROACH: 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. KEY FINDINGS: 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. IMPLICATIONS: 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. CONCLUSION: 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. [Author's abstract]