During the last quarter of the 20th century cannabis use has increased to become the most widely used illicit drug in the world1. It is well established that cannabis use is much higher amongst schizophrenic patients than in the general population2. Cannabis intoxication can lead to an acute transient psychotic episode and produce short-term exacerbations of pre-existing psychotic symptoms3–5, an association that has been confirmed through the experimental administration of tetrahydrocannabinol (THC)6, 7. Meta-analyses of prospective studies have found that cannabis use increases the likelihood of developing a psychotic illness by a factor of roughly two8–11. A dose response effect has been demonstrated12–14, and use in adolescence has been associated with the greatest risk15. Given the large health burden from schizophrenia and other psychotic disorders16, the view that cannabis use is a component cause of schizophrenia has heavily influenced discussion over the legislation surrounding cannabis use.
