According to the London Daily Mail, smoking a single joint of marijuana increases your risk of developing schizophrenia by 41 per cent. The Mail quoted Professor Robin Murray of the Institute of Psychiatry in London, who dutifully warned that the risk was perhaps even higher than that, due to the increasing use of what the newspaper termed “powerful skunk cannabis.” The skunk effect, said Murray, meant that the study’s estimate that “14 per cent of cases of schizophrenia in the UK are due to cannabis is now probably an understatement.”
Marjorie Wallace of the mental health charity SANE told BBC News: “The headlines are not scaremongering, but reflect a daily, and preventable, tragedy.”
Wow. As Gertrude Stein once put it, “Interesting if true.”
But it’s not true at all, of course. Or, to put it more accurately: If it were true, there is no way in hell the meta study under question could be used to prove it.
Speaking as a science journalist, this is the sort of thing than can really ruin your day.
As always, it helps to start with the original published article, a meta-analysis published in the British medical journal Lancet under the title, “Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review.” 2007 370: 319-28. Mark Hoofnagle, a MD/PhD Candidate in the Department of Molecular Physiology and Biological Physics at the University of Virginia, discussed the paper in depth on his Denialism Blog:
“First of all, the statement that ‘just one joint’ increases risk by 41% is absurd. The study here is of those who have tried marijuana once or more, not of people who have only tried it once. So already, the Daily Mail and every other news organization is way off. Second, I think we're ultimately seeing a post-hoc ergo propter hoc argument, and a dose-response that's more characteristic of the population studied than a real pharmacologic effect.”
--People who suffer from a mental illness do more drugs than “normal” people. They are a high-risk population when it comes to addiction. There are people who have a propensity for addiction, and people who do not. Many of those who do will get hooked, but this does not mean that everything which follows is a result of the drugs.
--Latent schizophrenics often suffer their first break while under the influence of psychoactive drugs. Pot, along with LSD, physical trauma, the death of a loved one, and other intense emotional events can all trigger a schizophrenic break in late adolescence. So naturally there would be a correlation.
--The assumption that pot causes susceptibility to mental illness, rather than the other way around, can’t be proven. Hoofnagle uses the example of cigarette smoking. Anyone who has researched schizophrenia, or been around schizophrenics, knows that almost all schizophrenics smoke. (It helps quell hallucinations). Most of them began to smoke before the onset of their illness. Using the assumptions of the current study, we could say that cigarette smoking is almost certain to cause schizophrenia. Correlation, as Hoofnagle reminds us, is not causation.
--Daily pot smokers confound such a study. Are some of them exhibiting symptoms of schizophrenia, or are they exhibiting the symptoms of chronic marijuana intoxication? If they quit smoking so much, would they stop acting so crazy?
--Comorbidity is exceedingly common in drug addicts and users. There is a well-documented causal connection between depression and the use of psychoactive drugs. People suffering from depression often resort to cannabis and other drugs as a form of self-medication. Again, the mental condition leads to the drug use, and not the other way around.
--Finally, where is the epidemic of schizophrenia caused by millions of people smoking marijuana for years? What field evidence can be drawn upon to support this remarkable conclusion?
To be fair to the authors, bets are hedged. In their conclusion, Moore, et.al. state: “The possibility that this association results from confounding factors or bias cannot be ruled out, and these uncertainties are unlikely to be resolved in the near future.”
Nevertheless, the authors go on to conclude that “We believe that there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life.”
At www.badscience.net, Ben Goldacre wryly notes that “You know when cannabis hits the news you’re in for a bit of fun…” Of 175 studies identified as potentially relevant, Goldacre maintains that only 11 papers, describing 7 discrete data sets, actually turned to be relevant for purposes of the study. If every assumption in the paper is taken to be correct, and causality is accepted, Goldacre calculated, about 800 cases of schizophrenia per year could be attributed to marijuana in the U.K. “But what’s really important,” Goldacre writes, “is what you do with this data. Firstly you can misrepresent it….not least of all with the ridiculous ‘modern cannabis is 25 times stronger’ fabrication so beloved by the media and politicians.”
As it happens, all of this comes at a time in Britain when efforts to reclassify cannabis are being hotly debated in the government. As propaganda, the report is useful, but as a means of clarifying the debate, it will only produce confusion and demagoguery.
--Moore, Theresa H.M., et. Al. “Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review.” Lancet. 2007 370: 319-28 http://www.thelancet.com/
--MacCrae, Fiona and Andrews, Emily. “Smoking just one cannabis joint raises danger of mental illness by 40%.” London Daily Mail. 26/07/07