Prohibitionists often cite the correlation between marijuana and schizophrenia as reason to oppose liberalization. However, correlation is not causation, and people with schizophrenia also use other psychoactive substances more than the general population. [1] So another possibility is that liability for schizophrenia causes a predisposition to use psychoactive substances. And that is exactly what studies that can determine causal relations find to be the best explanation for the marijuana-schizophrenia correlation.
This video reviews the MR studies examining causality between marijuana and schizophrenia.
Mendelian randomization (MR) is a method used by scientists to determine causal relationships between genetic liabilities and risk factors. In this case, the genetic liability is for schizophrenia and the risk factor is marijuana use. Because MR studies can determine causal relations, they are superior to the observational studies reporting a correlation between marijuana use and schizophrenia. Yet the MR studies on marijuana and schizophrenia get very little press compared to the less informative observational studies.
To date, there are three MR studies [2,3,4] examining causality between marijuana and schizophrenia. As seen in the video above, the evidence strongly supports liability for schizophrenia causing an increased likelihood of marijuana smoking. In contrast, evidence for marijuana causing schizophrenia is weak and uncertain, with weaker evidence that marijuana might actually decrease the risk of schizophrenia.
With strong evidence indicating that liability for schizophrenia causes an increased likelihood of marijuana use versus weak evidence for the reverse direction of causation, we have a best explanation for the correlation between marijuana and schizophrenia, an explanation that contradicts the arguments of marijuana prohibitionists.
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[1] Khokhar et al (2018): “47% of patients with schizophrenia have serious problems with drug or alcohol use during their lifetime compared to 16% of the general population. Regarding specific substances: tobacco, alcohol, cannabis and cocaine use disorders occur commonly in patients with schizophrenia.”
[2] Gage et al (2017). “Assessing causality in associations between cannabis use and schizophrenia risk: a two-sample Mendelian randomization study.”
[3] Vaucher et al (2017). “Cannabis use and risk of schizophrenia: a Mendelian randomization study.”
[4] Pasman et al (2018). “GWAS of lifetime cannabis use reveals new risk loci, genetic overlap with psychiatric traits, and a causal influence of schizophrenia.”
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