Sep 01, 2009
At least some in the international news media have belatedly discovered a study casting doubt on the purported link between marijuana use and schizophrenia. I say “belatedly” because the study was published online back in June, although the print version came out this month.
A group of British researchers examined a rather basic notion: If marijuana use causes schizophrenia, then a major increase in marijuana use should lead to an increase in schizophrenia diagnoses in the following years. In an enormous sample of some 600,000 Britons, no such thing occurred – indeed, a spike in marijuana use beginning in the mid-1970s was followed by rates of schizophrenia that either remained stable or declined.
Of course this is not the first time that a lack of connection between marijuana use rates and schizophrenia incidence has been noted in the scientific literature. For example, a 2006 review in the Canadian Journal of Psychiatry noted that “the treated incidence of schizophrenia did not obviously increase during the 1970s and 1980s when there were substantial increases in cannabis use among young adults in Australia and North America.” (Alas, that rather important discussion isn't mentioned in the summary linked above, which is all you can get for free).
Overall, the evidence strongly suggests that marijuana may worsen or trigger schizophrenia in a few individuals with a pre-existing vulnerability, but that it is not a significant cause of mental illness in healthy people. That rather nuanced reality tends to be a bit too complicated for many in the media.