Marijuana and psychosis: Real data, real bad

A just-published study in The Lancet:  “people who smoked marijuana on a daily basis were three times more likely to be diagnosed with psychosis compared with people who never used the drug. For those who used high-potency marijuana daily, the risk jumped to nearly five times.” - Christian Post

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Bert Perry's picture

As the article editorial linked notes, it's correlation and not causation, and one thing to note as well is that the article claims to have interviewed 901 out of about 1500 "first presentation" cases of psychosis in several European countries.  It's not entirely clear what a "site" is for monitoring--is it a nation's, a state's, or just a local mental hospital?--but given the prevalence of cannabis use (about 25% or so), it's not that many patients, and one would have to suggest that it's nothing like tobacco, where about ~ 800,000 people start smoking each year in the U.S., and about 480,000 of them eventually die from using it.

Regarding the notion that you can't demonstrate causality, well, not quite.  It's hard work--tobacco use, with a relative risk of lung cancer of something like 40 (vs. 3-5 here), took decades--but it can be done.  Some thoughts as to how:

  • Longitudinal study of the onset of psychotic symptoms vs. cannabis use.  
  • Study of what happens to psychotic symptoms when cannabis use is stopped.  (assuming it's not permanent damage)
  • Brain studies on cannabis vs. non-cannabis users

For now, I think it's safe to say that there is a relative risk that's interesting, and that those who might use could be notified of it--perhaps not to use, or perhaps to simply take a look at other measures of mental health.  

Aspiring to be a stick in the mud.