Physician's First Watch for July 27, 2007

David G. Fairchild, MD, MPH, Editor-in-Chief

Cannabis Use Linked to Later Psychosis

West Nile Virus Rate Seems Higher Than in Previous Years

Cannabis Use Linked to Later Psychosis

A Lancet meta-analysis on the late effects of cannabis use finds that the drug is associated with a 40% increase in the risk for psychotic symptoms, with more use leading to greater risk.

The authors assessed the studies with particular attention to bias and confounding factors, such as intoxication effects and subjects' having experienced psychosis at baseline. In the seven studies evaluated for a link with psychosis, the odds ratio for psychosis in those who had ever used cannabis versus nonusers was 1.41. Among the most frequent users, the odds ratio was 2.09.

The authors say the question is not so much whether cannabis causes psychosis as it is "whether the evidence ... now available can justify policy implications, such as public education campaigns to alert people to the possible risks."

A commentator calls the study "the most comprehensive meta-analysis to date" and concludes that cannabis' "long-term hazardous effects ... with regard to psychosis seem to have been overlooked."

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