Cannabis and psychosis

Cannabis is the most widely used illegal drug in the world and there is increasing evidence that it can cause psychosis in people with no previous history of mental illness. The use of cannabis is particularly common in the age group most at risk for the onset of psychosis, 15-30. But not all heavy users of cannabis go on to develop psychosis and not all people who develop psychosis have used cannabis. One factor behind these differences is thought to be people's underlying proneness to psychosis, or schizotypy. Researchers from University College London compared 140 cannabis users with 144 non-users. The cannabis users were asked about their psychosis symptoms while they were intoxicated and when they were free of the drug with the non-users being asked about their psychosis symptoms at the same time. Both groups were given a questionnaire to assess their proneness to psychosis. Smoking cannabis was found to lead to a marked increase in psychosis symptoms and the more prone to psychosis someone was the worse their symptoms were when they took cannabis.

Mason, O. ... [et al] - Acute cannabis use causes increased psychotomimetic experiences in individuals prone to psychosis Psychological Medicine June 2009, 39(6), 951-956