A meta-analysis of 32 studies in the July 28 issue of The Lancet finds that pot smoking is associated with a 41 percent increase in the risk of "any psychotic outcome." According to the Daily Mail, this means "smoking just one cannabis joint raises danger of mental illness by 40%." A.P. is slightly more cautious, reporting that "using marijuana seems to increase the chance of becoming psychotic." Time ran the A.P. story under the headline "Marijuana 'Raises Psychosis Risk.'" A.P. introduces a pretty important note of caution in the fifth paragraph:
The researchers said they couldn't prove that marijuana use itself increases the risk of psychosis, a category of several disorders with schizophrenia being the most commonly known.
There could be something else about marijuana users, "like their tendency to use other drugs or certain personality traits, that could be causing the psychoses," Zammit said.
By contrast, the Daily Mail waits until the 29th (and last) paragraph to note that "others questioned the link, pointing out there has been little change in rates of schizophrenia in recent years despite the rise in cannabis use and the increasing strength of the drug." But the paper more than makes up for that concession with a sidebar on "three heavy drug users and their horrific killings" that Harry Anslinger would have envied. The best of the three:
Son of a nurse at Broadmoor Thomas Palmer butchered two of his friends during a woodland walk after his mind was warped by smoking skunk—a particularly potent form of cannabis.
Then aged 18, he virtually beheaded 16-year-old Steven Bayliss and repeatedly stabbed Nuttawut Nadauld, 14, near their homes in Wokingham, Berkshire in September 2005.
Palmer had started using the drug at 14. He told doctors he had not been smoking on the day of the killings but admitted to using skunk regularly in the weeks before the brutal attack.
Scientifically, correlational studies like those analyzed in the Lancet article are superior to such gory anecdotes, but they cannot answer the crucial question of why marijuana users are somewhat more likely to be diagnosed as psychotics. As the authors note, "The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies such as those reviewed here." By the same token, we can't say for sure that marijuana doesn't trigger or exacerbate psychological problems in certain vulnerable individuals. But as Lester Grinspoon and James Bakalar pointed out in connection with psychedelics, the same could be said of any emotionally powerful experience.
Furthermore, the risk is much lower than the impressive-looking 40 percent figure suggests. The overall incidence of schizophrenia, A.P. says, is "less than 1 percent," in which case smoking pot would raise it to less than 1.4 percent. That's assuming the relationship is causal, which it might not be, and that the risk is random, which it almost certainly isn't.
Addendum: Alex Duncan points out that reason contributor Maia Szalavitz has a post on the study at the STATS site, emphasizing the lack of a correlation between marijuana use rates and the incidence of schizophrenia.
[Thanks to Dan Greene and jimmydageek for the tip.]