The Link Between Psychotic Disorders and Substance Use | Psychiatric TimesThe most robust body of evidence for the relationship between the onset of psychotic disorders and drug use comes from the Swedish conscript studies.12,13 Baseline drug use information was taken at intake to the army for these studies. The authors found that cannabis use, and to a lesser extent amphetamine use, predicted onset of psychotic disorder later in adulthood. The explanatory variable was age at use, with younger age predicting psychotic disorders. This relationship did not exist for other drugs used, such as cocaine. A more recent cohort study in New Zealand confirmed the relationship between cannabis use and the onset of psychotic disorders.14 This study demonstrated that younger age of first cannabis use predicted younger age of psychotic disorder onset in individuals with the Val/Val genotype for COMT on the Val158Met polymorphism (a gene that codes for the activity level of an enzyme involved in the breakdown of dopamine).