In March 2019 researchers from the UK, Italy, France, Spain, The Netherlands and Brazil published the results of their study to assess the effect of cannabis use on the risk of psychotic disorders. 901 individuals, aged 18-64 years, who had experienced their first episode of psychosis from 11 sites across Europe and Brazil were included in the study. Also involved were 1237 individuals from the same local populations who acted as a control group. Information on the expected concentration of the cannabis used by individuals was collected using Europe-wide and national data. The types of cannabis used were then divided into two categories, namely low potency and high potency. Results showed that daily cannabis use was associated with an increased risk of a psychotic disorder when compared with never users, with daily users of the high-potency types of cannabis having a five times increased risk. A further analysis revealed that if high-potency cannabis was no longer available, there would be an estimated 12% reduction in the number of psychotic episodes across the 11 sites. However, when individual cities were assessed the reduction in number of psychotic episodes rose to 30% for London and 50% for Amsterdam.
Di Forti M et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. Lancet Psychiatry. 2019 Mar 19. [Epub ahead of print]