There has been much interest in, and debate about, the links between cannabis use and psychosis. Cannabis intoxication can produce transient psychotic and affective experiences and can have detrimental effects on motivation and memory. Yet people with psychosis also report that it can reduce their anxiety and make them more sociable. A review of 13 studies into the links between cannabis and psychosis carried out by researchers at Bristol University found that cannabis use was consistently associated with relapse and non-adherence to medication. However, the researchers also found that few of the studies took into account how ill people were to begin with and the influence of alcohol or other factors on people's psychosis. Taking these other factors into account resulted in a substantial weakening of the link between cannabis and psychosis.
Zammit, Stanley ... [et al] - Effects of cannabis use on outcomes of psychotic disorders: systematic review British Journal of Psychiatry November 2008, 193(5), 357-363