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Cannabis and Psychosis: What Degree of Proof Do We Require?

  • Robin M. Murray
    Correspondence
    Address correspondence to Robin Murray, Department of Psychosis Studies, Institute of Psychiatry, Psychology and Neuroscience, King’s College, De Crespigny Park, London SE5 8AF, United Kingdom.
    Affiliations
    Department of Psychosis Studies, Institute of Psychiatry, Psychology and Neuroscience, King’s College, London, United Kingdom
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  • Marta Di Forti
    Affiliations
    Department of Psychosis Studies, Institute of Psychiatry, Psychology and Neuroscience, King’s College, London, United Kingdom
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      It has been almost 30 years since Andreasson et al. demonstrated in 1987 an association between cannabis use and subsequent onset of schizophrenia [reviewed in (
      • Gage S.H.
      • Hickman M.
      • Zammit S.
      Association between cannabis and psychosis: Epidemiologic evidence.
      )]. There was a 15-year gap before any replications of their results were attempted. However, as Gage et al. (
      • Gage S.H.
      • Hickman M.
      • Zammit S.
      Association between cannabis and psychosis: Epidemiologic evidence.
      ) point out, many longitudinal prospective studies have now been published. All four investigations into the relationship between cannabis use and subsequent psychotic illness reported a significant association; three studies that focused on cannabis use and psychotic experiences also reported a significant association, and a further three studies showed such an association, which became attenuated after adjustment for confounders.
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