The mainstream media was abuzz this week promoting an age-old claim: Smoking marijuana makes you crazy.

“Psychosis five times more likely for cannabis users: study,” a wire story from Agence France-Presse declared. The UK Mail on Sunday expressed similar alarm, declaring, “Scientists show cannabis TRIPLES psychosis risk.” Somewhat surprisingly, it was Fox News that took the most reserved approach, announcing “Smoking high-potency marijuana may cause psychiatric disorders.”

So what was the source of this latest round of sensational headlines? Writing in the February 18 edition of the British journal The Lancet Psychiatry, investigators assessed rates of cannabis use in a cohort of South London first-episode psychosis patients versus pot use frequency in a similar group selected from the general population. Authors reported that subjects with psychosis were more likely to recall having used “skunk-like cannabis” daily as compared to controls, whereas those participants who reported primarily consuming hashish possessed no elevated risk of having such a diagnosis. (Researchers defined so-called skunk marijuana as cannabis possessing THC concentrations above 15 percent. Of course, since cannabis is illegal in Britain and the weed obtained on the black market is not subject to analytical potency testing, it is unclear how subjects in the study—or its authors—knew whether participants were consuming supposed "high-potency" herb or just regular, plain old weed.)

Authors also acknowledged that nearly half of patients with first-episode psychosis reported having smoked 100 cigarettes or more. These patients were also more likely to be black and were less likely to have completed high school compared to controls, though, predictably, none of these observed associations triggered international headlines.

“This paper suggests that we could prevent almost one-quarter of cases of psychosis if no one smoked high-potency cannabis,” senior researcher Sir Robin Murray, a psychiatric research professor at King’s College, said in the news release accompanying the paper’s publication. Murray’s statement, publicized widely by the mainstream media, was not only hyperbolic, but it also showed an apparent willful disregard for the scientific rule: association does not equal causation. In fact, patients with a variety of psychiatric disorders tend to consume numerous intoxicants, including pot but also tobacco, at elevated rates compared to the general population – many of whom are likely engaging in this behavior as a form of self-medication. But this overlap is hardly evidence that one behavior causes the other.

What’s Old Is New Again

Think you have heard these pot-drives-you-insane claims before? You have. In 2007, The Lancet published a meta-analysis similarly alleging, “People who have ever used cannabis, on average, have about a 40 percent increased risk of developing psychotic illness later in life compared with people who have never used cannabis." That the study’s authors cautioned that such an association "does not necessarily reflect a causal relation" between pot smoking and mental illness went largely unreported.

Yet, in the following years since, numerous (though far less publicized) studies have come to light downplaying the likelihood that cannabis use is a direct cause of psychiatric disorders like schizophrenia. Specifically, a 2009 paper in the journal Schizophrenia Research compared trends in marijuana use and incidences of schizophrenia in the United Kingdom from 1996 to 2005. Authors reported that "incidence and prevalence of schizophrenia and psychoses were either stable or declining" during this period, even though pot use among the general population was rising. They concluded: "This study does not therefore support the specific causal link between cannabis use and incidence of psychotic disorders. … This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence."

Similarly, a 2010 review paper published by a pair of British scientists in the journal Addiction reported that clinical evidence indicating that use of the herb may be casually linked to incidences of schizophrenia or other psychological harms is not persuasive. Authors wrote: "We continue to take the view that the evidence that cannabis use causes schizophrenia is neither very new, nor by normal criteria, particularly compelling. … For example, our recent modeling suggests that we would need to prevent between 3000 and 5000 cases of heavy cannabis use among young men and women to prevent one case of schizophrenia, and that four or five times more young people would need to avoid light cannabis use to prevent a single schizophrenia case. … We conclude that the strongest evidence of a possible causal relation between cannabis use and schizophrenia emerged more than 20 years ago and that the strength of more recent evidence may have been overstated."

More recently, researchers at Harvard University released a study further rebutting this allegation. Writing in 2013 in Schizophrenia Research, investigators compared the family histories of 108 schizophrenia patients and 171 individuals without schizophrenia to assess whether youth cannabis consumption was an independent factor in developing the disorder. Researchers reported that a family history of schizophrenia increased the risk of developing the disease, regardless of whether or not subjects consumed weed as adolescents. They concluded: “The results of the current study, both when analyzed using morbid risk and family frequency calculations, suggest that having an increased familial risk for schizophrenia is the underlying basis for schizophrenia in these samples and not the cannabis use. While cannabis may have an effect on the age of onset of schizophrenia it is unlikely to be the cause of illness.”

In fact, some researchers speculate that specific cannabinoids, such as cannabidiol (CBD), may even be efficacious in treating symptoms of psychosis. According to a review published in the January 2014 issue of the journal Neuropsychopharmacolog y: “CBD has some potential as an antipsychotic treatment. … Given the high tolerability and superior cost-effectiveness, CBD may prove to be an attractive alternative to current antipsychotic treatment.” Specifically, a 2012 double-blind, randomized placebo-controlled trial assessing the administration of CBD versus the prescription anti-psychotic drug amisulpride in 42 subjects with schizophrenia and acute paranoia concluded that two substances provided similar levels of improvement, but that cannabidiol did so with far fewer adverse side effects.

Case reports in the scientific literature also indicate that some patients turn to cannabis for subjective benefits, though other studies indicate that pot use may exacerbate certain symptoms in patients with psychiatric disorders. Nonetheless, even a recent paper summarizing the “adverse health effects of recreational cannabis use” acknowledges, “It is difficult to decide whether cannabis use has had any effects on psychosis incidence, because even if a relationship were to be causal, cannabis use would produce a very modest increase in incidence.”

The Bottom Line? Mental Health Concerns Don’t Justify Criminalization

Is it possible that the habitual use of high-potency cannabis may potentially aggravate or even trigger psychiatric episodes in subjects predisposed to certain mental illnesses? Yes. However, such concerns are not persuasive justifications for continuing cannabis criminalization. Just the opposite holds true. There are numerous adverse health consequences associated with alcohol, tobacco and prescription drugs, all of which are far more dangerous and costlier to society than cannabis. It is precisely because of these consequences that these products are legally regulated and their consumption is restricted to specified setting and to certain consumers (the most vulnerable of which, such as pregnant women, are often explicitly warned of the drug’s potential adverse effects in this population). It is time to once and for all ended society’s nearly century-long love affair with reefer madness and applied these same common sense principles to cannabis.