The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study

The Lancet Psychiatry, 19 March 2019

This large multicentre case-control study conducted across 11 sites in Europe and Brazil found that daily cannabis use was associated with increased odds of psychotic disorder compared with people who had never used cannabis, increasing to nearly five-times increased odds for daily use of high-potency types of cannabis.

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‘Skunk-like cannabis’ increases risk of psychosis, study suggests

BBC News, 16 February 2015

Smoking potent cannabis was linked to 24% of new psychosis cases analysed in a study by King’s College London.  The research suggests the risk of psychosis is three times higher for users of potent “skunk-like” cannabis than for non-users….

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Alcohol confounds relationship between cannabis misuse and psychosis conversion in a high-risk sample

Acta Psychiatrica Scandinavica, January 2015

Objective:
Cannabis use has been examined as a predictor of psychosis in clinical high-risk (CHR) samples, but little is known about the impact of other substances on this relationship.

Method:
Substance use was assessed in a large sample of CHR participants (N = 370, mean age = 18.3) enrolled in the multisite North American Prodrome Longitudinal Study Phase 1 project. Three hundred and forty-one participants with cannabis use data were divided into groups: No Use (NU, N = 211); Cannabis Use without impairment (CU, N = 63); Cannabis Abuse/Dependence (CA/CD, N = 67). Participants (N = 283) were followed for ≥2 years to determine psychosis conversion.

Results:
Alcohol (45.3%) and cannabis (38.1%) were the most common substances. Cannabis use groups did not differ on baseline attenuated positive symptoms. Seventy-nine of 283 participants with cannabis and follow-up data converted to psychosis. Survival analysis revealed significant differences between conversion rates in the CA/CD group compared with the No Use (P = 0.031) and CU group (P = 0.027). CA/CD also significantly predicted psychosis in a regression analysis, but adjusting for alcohol use weakened this relationship.

Conclusion:
The cannabis misuse and psychosis association was confounded by alcohol use. Non-impairing cannabis use was not related to psychosis. Results highlight the need to control for other substance use, so as to not overstate the cannabis/psychosis connection.

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