Does smoking pot make people crazy, or do lots of crazy people smoke pot? That’s the question raised by the findings of a just-published study sponsored by the Canadian Centre for Addiction and Mental Health.

It’s hardly the first time that society has pondered this issue. Allegations that cannabis triggers violent or psychotic behavior were the basis for the federal government’s successful “Reefer Madness” campaign in the 1930s. Hindsight, of course, later showed these claims to be categorically false. Nevertheless, over the past decade, concerns regarding marijuana’s role as a potential underlying component in various forms of mental illness, particularly schizophrenia, have once again surfaced, largely fueled by prohibitionist forces in the United Kingdom and the U.S. As in the past, however, the hard science in support of such claims remains largely specious.

Despite a spate of recently published peer - reviewed papers on the subject, investigators’ findings remain for the most part contradictory, raising far more questions than answers.

Such is the case with the latest data compiled by Canadian scientists and published online in the journal Scientific Psychiatry. Investigators at the Centre for Addiction and Mental Health and the University of Toronto analyzed data gathered from face - to - face interviews with a nationally representative cohort of over 43,000 U.S. respondents ages 18 and older. Using structured questionnaires, the researchers assessed the association between cannabis use and the prevalence of various mental illnesses, including depression, anxiety and personality disorders. 

“The odds ratio for cannabis use among individuals with 12 - month mental illness vs. respondents without any mental illness was 2.5,” the authors concluded, noting that the plant was “particularly associated with bipolar disorder, substance use disorders and specific (anti-social, dependant and histrionic) personality disorders.”

But association is not causation, warns Mitch Earleywine, PhD, author of the book, Understanding Marijuana: A New Look at the Scientific Evidence. (He is also a member of NORML’s board of directors.) Earleywine notes that patients with mental illness typically consume various psychotropic substances at rates far greater than those of the general population. Indeed, a 2009 population-based assessment of US and Australian respondents reported that patients with affective disorders were more than twice as likely to smoke cigarettes than were adults with no prior history of such illness -- but no one is suggesting that puffing Marlboros triggers mental instability.

Reviews of the cannabis - use data from around the globe indicate that pot doesn’t do so either. For example, people are no more likely to be diagnosed with mental illness in the United States and Canada -- two nations with among the highest rates of cannabis use by their populations -- than they are in Sweden or Japan, where self-reported marijuana use is comparatively low.

Furthermore, the increased popularity of cannabis in recent decades, both here and abroad, has not been associated with a parallel increase in the prevalence of mental disorders. Specifically, a 2009 study authored by researchers at Keele University Medical School in Britain compared trends in marijuana use and the incidence of serious mental illness in the United Kingdom for a 10 - year period from 1996 to 2005. Investigators found that the “incidence and prevalence of schizophrenia and psychoses were either stable or declining” during this period, even as the use of cannabis among the general population was rising.

So if cannabis use isn’t a cause of mental illness, why do so many people with psychiatric disorders report using pot? The likely explanation for this association, says Earleywine, is that many patients are simply self - medicating with cannabis. Pot is well known to be a safe and effective mood enhancer, and cannabidiol, a key component of the plant, possesses documented anti-anxiety properties. A 2012 study published in the scientific journal, Psychiatry Research reported that bipolar patients with a history of marijuana use performed better on a series of neurocognitive tests than bipolar patients with no history of pot use. Similar findings have been reported among patients with schizophrenia.

Even more notable, a 2012 paper authored by researchers at the University of Maryland found lower mortality rates among cannabis - using patients diagnosed with schizophrenia, psychoses and related disorders as compared to nonusers. A 2005 review published in the Journal of Psychopharmacology called for the clinical study of cannabinoids as an adjunctive treatment for patients with various mental - health issues, acknowledging that the compounds “exert sedative, hypnotic, antidepressant, [and] antipsychotic … effects.” As Earleywine concludes: “Cannabis doesn’t make people mentally ill, but a lot of mentally ill people use cannabis.”

Other recent studies have questioned the supposed association between cannabis use and mental illness altogether. Writing in the journal Addiction in 2010, researchers at the University of Bristol’s Department of Social Medicine concluded that clinical evidence linking pot use with the incidence of schizophrenia or other psychological diseases was “overstated.” Most recently, a 2013 paper in the same journal, authored by investigators at the Netherlands Institute of Mental Health and Addiction, concluded: “Our data suggest that the prevalence of internalizing disorders is no higher in non - dependent frequent cannabis users than in the general population, suggesting that the mental health risks of frequent non - dependent cannabis use may have been overestimated in previous studies.”

Ultimately, however, the re-emerging claims about cannabis and its harmful effects on mental health will likely do little to stave off the increasing momentum of public sentiment in favor of the plant’s legalization and regulation. Unlike the 1930s, when such scare tactics spurred calls for criminalization, today’s claims of cannabis use causing psychotic behavior appear to carry little weight with a voting public that’s become justifiably weary of pot prohibition.

Paul Armentano is deputy director of NORML.