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Heavy cannabis use contributes to the incidence of Psychotic Disorders in Amsterdam and across Europe

Marta Di Forti  

Abstract
Background Cannabis use is associated with increased risk of later psychotic disorder but whether it has an impact on incidence of the disorder remains unclear. We aimed to: 1) identify patterns of cannabis use with the strongest impact on odds of psychotic disorder across Europe, 2) whether differences in such patterns contribute to variations in the incidence rates (IRs) of psychotic disorder.

Methods We analysed data from 901 first episode psychosis patients aged 18–64 years recruited from 11 sites across Europe, and 1237 controls representative of the local populations.

Results Daily cannabis use was associated with increased odds of psychotic disorder, compared to never used; increasing to nearly 5-fold for daily use of high potency types of cannabis. The IRs for psychotic disorder were positively correlated with the prevalence in controls across the 11 sites of a) use of high-potency cannabis (r=0‧7; p=0‧02) and b) daily use (r=0‧8; p=0‧01).

Conclusions Differences in frequency of daily cannabis use, and in use of high potency cannabis across 11 sites, contributed to the striking variation in the incidence of psychotic disorder.