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Research Summary

Cannabis Use May Lead to Later Psychosis

Cannabis intoxication can produce transient psychotic and affective symptoms. Whether cannabis use leads to the development of subsequent psychosis or affective symptoms is less clear. After searching the medical literature, researchers systematically identified and reviewed 35 longitudinal studies that examined the incidence of psychosis or affective symptoms among cannabis users.

  • Subjects who had ever used cannabis had a higher risk of developing psychosis than did nonusers (pooled adjusted odds ratio [OR], 1.4).
  • All 6 studies reporting on cannabis frequency found a dose-response relationship between cannabis use and psychosis (pooled adjusted OR, 2.1 for the most frequent cannabis users versus nonusers).
  • In the 8 studies that examined cannabis use frequencies, frequent cannabis users had a higher risk of developing depressive symptoms than did nonusers (pooled adjusted OR, 1.5).
  • In studies that examined a depression diagnosis, suicidal ideation or attempts, or anxiety outcomes, there was no significant association between cannabis use and these outcomes.

Comments:

The positive association between cannabis use, particularly frequent use, and the development of psychosis is consistent across longitudinal studies, whereas the association between cannabis use and affective symptoms is largely mixed. Clinicians should inform patients that cannabis use may increase the risk of future psychotic illness.

Alexander Y. Walley, MD, MSc

Reference:

Moore THM, Zammit S, Lingford-Hughes A, et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet. 2007; 370(9584):319–328.


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