Why Does the Myth of Marijuana and Schizophrenia Persist Despite Numerous Debunkings?

José Antonio Alzate y Ramírez, an eminent Mexican scientist, studied the effects of marijuana on human subjects and became quite alarmed at what he saw. “There is no doubt that the health of the population is a central concern here,” he wrote. “The violent effect of the narcotics proves this sufficiently; it has not been but a few months since a person to whom they administered the drug, I do not know for what purpose, in perhaps too great a dose, lost his mind.” The year was 1772.*

The idea that the use of marijuana causes some form of insanity has persisted for centuries; the notion seems to crop up anywhere that European colonizers encountered indigenous populations using the drug. For example, in British-occupied India, the Englishman George Francis William Ewens wrote in 1904 that “[t]he smallest practical experience of insanity among natives of India is sufficient to convince one that... excessive indulgence in any form of hemp drugs is a very frequent cause of mania.” Ewens reported “a widespread belief... among European medical men practicing among them, that an excess in the use of these intoxicants... is the cause of an enormous proportion of the cases of insanity occurring in the country.”

In the 1950s, Frances Ames studied the effects of “dagga” (an indigenous word for marijuana) on native populations in South Africa and compared the effects to schizophrenia. Then, in the 1960s, American psychiatrists John A. Talbott and James W. Teague traveled to Vietnam to study the effects of cannabis on the native population there. They concluded:

“Cannabis derivatives, as a causal or precipitating agent, should be considered whenever a young person presents with an acute toxic psychosis with paranoid features... the physician must be alert to the possibility of marihuana psychosis in cases resembling acute schizophrenic reaction, acute paranoid psychosis, or acute toxic-metabolic psychosis.”

The idea has persisted despite numerous debunkings.

As early as 1894, the Indian Hemp Drugs Commission report concluded that alarmist tales of cannabis insanity in the Indian subcontinent were exaggerated and that moderate use of the drug was relatively harmless. A 1925 report by the US Army in the Panama Canal Zone reached a similar conclusion:

“There is no evidence that marihuana as grown and used here is a 'habit-forming' drug in the sense in which the term is applied to alcohol, opium, cocaine, etc.,” the Army reported, “or that it has any appreciably deleterious influence on the individuals using it.”

A 1937 report to the League of Nations agreed.

“It is true that some directors of lunatic asylums in Egypt and British India have estimated the number of psychoses due to the abuse of hashish at 15% or even 30% of all the cases admitted in the course of a year,” the report conceded, “but it is generally held by the experts of Western countries that this percentage must be exaggerated, and that many cases of schizophrenia are dissimulated under the erroneous diagnosis of toxic insanity.”

Similar conclusions were also reached by a 1975 Jamaican study, and a 1980 study in Costa Rica. The 1999 Institute of Medicine Report noted that “for some patients – particularly older patients with no previous marijuana experience – the psychological effects are disturbing” but also described these effects as “generally more severe for oral THC [i.e., Marinol] than for smoked marijuana.”

The report concluded that the psychological effects of cannabis, far from causing incurable psychosis, “are potentially undesirable for certain patients and situations and beneficial for others.”

Now, the most powerful evidence against the supposed marijuana-insanity link has come forth, with the publication of a new Harvard study showing that evidence of increased prevalence of schizophrenia among people who smoked pot in their teens can be explained by genetic and other familial factors instead of any causal relationship between pot and mental illness.

Why, then, does the myth of marijuana and mental illness still persist? Historically, cultural differences surely had much to do with the mistake of science; rituals using cannabis in exotic places like Africa, India and the indigenous tribes of Mexico may have appeared so strange to European eyes as to evoke thoughts of psychosis in the observer. Such first impressions can have a pervasive effect, as shown by the example of marijuana insanity stories from Mexico influencing “reefer madness” dialogue in the United States. But a more provocative theory, advanced by Dr. Andrew Weil (yes, that Dr. Andrew Weil) in the New England Journal of Medicine is that doctors looking to find cannabis psychosis create a kind of self-fulfilling prophecy, through which their leading questions of interviewed pot smokers under the influence provoke a paranoid panic reaction which is often mistaken for psychosis. Stoned patients become afraid that they might have a mental illness, the drug experience amplifies that belief, and the doctor goes home, congratulating himself on finding what he was looking for.

Confirmation bias is a hell of a drug. Perhaps that's why one still finds serious-sounding people repeating old reefer madness myths, right up to the present day.

*This quote and many other historical facts contained in this article are pulled from Home Grown: Marijuana and the Origins of Mexico's War on Drugs by Isaac Campos. University of North Carolina Press, 2012.
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