Cannabis and Psychosis Risk — MRCPsych Paper B MCQ
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Correct answer: C — Cannabis use is associated with an increased risk of psychotic disorder, with greater risk from more frequent use and higher-THC products
Explanation lettering: C = shown as A · E = shown as B · B = shown as C · A = shown as E
Cannabis use is associated with an increased risk of psychotic disorder, and the association shows a dose-response pattern: more frequent use and higher-THC products confer greater risk. The EU-GEI study found the highest odds among people reporting daily use of high-potency cannabis. Cannabis is nevertheless neither necessary nor sufficient for psychosis, so observational evidence cannot prove that it caused this particular patient's episode. Vulnerability may modify individual risk, but the association is not restricted to people with specified genetic variants. Reverse causation and residual confounding remain relevant limitations but do not fully account for the observed evidence, making A incorrect. C wrongly restricts the relationship to intoxication. D is false because plant cannabis is also associated with psychosis. E contradicts the observed frequency- and potency-related gradients.
Reference: 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. The Lancet Psychiatry. 2019. https://pubmed.ncbi.nlm.nih.gov/30902669/