The claim that cannabis causes or precipitates psychosis has long been a foundation for opposing cannabis. Yet with hundreds of millions of dollars invested and over a century of dedicated research, the argument remains weak. A brief article by UK researchers attempts to describe why. Population studies clearly demonstrate that cannabis use does not simply cause schizophrenia — there is little correlation between rates of cannabis use and schizophrenia in the population (these are called epidemiological studies). But there is consistently an association between cannabis use and the development of psychosis. Three common explanations are (1) cannabis might precipitate schizophrenia in vulnerable populations, (2) people use cannabis to self-medicate, even before a diagnosis of schizophrenia, and (3) certain genetic or environmental factors may promote both cannabis use and schizophrenia. Although the authors do not describe this directly, the first point is reminiscent of arguments from the anti-vaccination campaign — when epidemiological research indicated that vaccination does not correlate well with the rate of autism, the argument moved to “it can precipitate autism in vulnerable individuals,” a claim that is much harder to test. This, applied to cannabis, is essentially the argument of (1). The authors instead highlight how research methodology has precluded an ability to prove or disprove explanations (1) and (2). Research even sometimes suggests that schizophrenia predicts cannabis use, rather than the other way around. The high prevalence of tobacco use is another major confounding factor in cannabis research. Finally, the authors conclude their commentary by emphasizing demographic limitations. Women are often excluded from these studies — most inquiries into this topic have studied only men from western countries. This is a major problem that pervades medical research.
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