Cannabis use and the risk of developing a psychotic disorder

WE BRIEFLY REVIEW THE EVIDENCE THAT CANNABIS USE IN ADOLESCENCE AND YOUNG ADULTHOOD IS A CONTRIBUTORY CAUSE OF SCHIZOPHRENIFORM PSYCHOSES, BY SUMMARISING LONGITUDINAL STUDIES THAT: a) have examined relationships between cannabis use and the risk of psychosis or psychotic symptoms; and b) have controlled for potential confounders, such as other forms of drug use and personal characteristics that predict an increased risk of psychosis. There is now reasonable evidence from longitudinal studies that regular cannabis use predicts an increased risk of schizophrenia and of reporting psychotic symptoms. These relationships have persisted after controlling for confounding variables such as personal characteristics and other drug use. The relationships did not seem to be explained by cannabis being used to self-medicate symptoms of psychosis. A contributory causal relationship is biologically plausible because psychotic disorders involve disturbances in the dopamine neurotransmitter system with which the cannabinoid system interacts, as has been shown by animal studies and a human provocation study. We briefly explore the clinical and public health implications of the most plausible hypothesis, that cannabis use precipitates schizophrenia in persons who are vulnerable because of a personal or family history of schizophrenia.