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By PHILIP M. BOFFEY
August 4, 2014 5:33 pm

People who oppose legalization of marijuana routinely raise the possibility that marijuana may cause or exacerbate episodes of psychosis, including schizophrenia. This is hardly settled science, but it’s not necessarily bogus, either.

The Federal government’s most recent evaluation of the health hazards of marijuana was written by experts at the National Institute on Drug Abuse and published in the New England Journal of

Medicine in June. In describing the effects of long-term or heavy use, it found an increased risk of chronic psychosis disorders, including schizophrenia, in persons with a predisposition to such disorders. It also warned that marijuana could exacerbate the course of illness in patients who have schizophrenia.

But the NIDA experts acknowledged that it is difficult to establish that marijuana actually caused these bad outcomes (as opposed to simply being “linked with” or “associated with” them) because many other factors may have caused the mental illness.

Another recent review of the evidence, written by experts at the Yale University School of Medicine and published in May in the journal “Frontiers in Psychiatry,” judged it “likely” that cannabis is an important component cause of psychotic disorders. The comprehensive 45-page review noted that some analyses have found that 8 to 14 percent of all schizophrenia cases may be due to cannabis use. But the review acknowledged that such estimates don’t mesh with this fact: a huge surge in cannabis consumption over the past four decades has not been accompanied by a commensurate increase in schizophrenia rates. The review also noted that a biologically plausible mechanism by which cannabis can increase the risk of psychotic disorders has yet to be established.

A 2010 paper by Australian researchers concluded that existing data from a slew of major studies can’t determine whether cannabis use can cause serious psychotic disorders that would otherwise not have occurred.

Schizophrenia is a relatively rare condition that afflicts about 1 percent of the American adult population. Although some people with a predisposition or existing disease may be adversely affected by marijuana use, the vast majority of individuals who consume cannabis do not experience any kind of psychosis. They need to identified and protected against harm but the risk to this relatively small group is not sufficient reason to bar marijuana use for everyone else.