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.