Reefer Madness is often invoked as a vignette of the hysteria that pervades conversations about drug use. In the cult classic film, cannabis use results in the characters experiencing hallucinations and psychosis that lead to several violent deaths. While most people now laugh at the film as a campy relic of yesteryear, make no mistake, the argument about cannabis’ association with psychosis is alive and well, albeit with less of the theatrics.  

A recent Cambridge University study using data from almost seven million people assessed the relationship between cannabis use disorder and schizophrenia. The study’s scale and depth make it worth paying attention to. The authors found that males between the ages of 16 and 20 with cannabis use disorder and regular to high cannabis use are at greater risk of triggering schizophrenia than females of the same age. The risk of schizophrenia does not differ between older males and females. Although the study’s design cannot prove that cannabis use disorder causes schizophrenia, the authors estimate that in 2021, if you assume a causal relationship, about 15 percent of schizophrenia diagnoses among males could have been prevented if cannabis use disorder was not present.

It is well known that extensive substance use can alter adolescent and young adult brain development to a greater degree than adult brains. Because of this, youth substance use is often framed as more risky or harmful than adult substance use. This is one reason why minimum age of purchase laws exist. As the Cambridge study of cannabis use disorder and schizophrenia shows, there are often different levels of risk among populations. For this reason, it is important that public health education and communication provide a nuanced description of risk for different groups.

Although it is impossible for people to know their individual, long-term risk or susceptibility to the negative or positive effects of cannabis with certainty, population-based generalizations can help educate people about their potential risks. Taking a harm reduction approach to cannabis that accounts for individual- and population-level risk is likely the most effective way to do this. Harm reduction does not ignore or minimize the risks associated with a behavior, and it recognizes that individuals may decide to engage in behaviors despite those risks. Harm reduction aims to give people the tools and information they need to minimize the risks associated with potentially harmful behaviors.

For cannabis and schizophrenia, since different demographics appear to have different risks, minimizing risks and reducing harm will look different for different groups. As such, educating young people about the potential risks of cannabis use, without fearmongering or exaggerating, is important. However, for adults, there are fewer (but not zero) risks associated with cannabis use. For adults, the balance of risk to reward may even skew toward greater rewards in some cases, such as for people who find benefits from medicinal cannabis. Thus, some strategies might include encouraging people to use cannabis with lower THC content, since some evidence suggests that higher THC content may increase risk of psychosis, and to consume cannabis in careful and intentional doses to ensure that the desired effects are achieved with the lowest possible dose.

Doubtlessly, substance use has associated risks, and these risks are not the same for every individual or every population. More high-quality studies, like the one discussed here, are necessary to better understand these differences so that policymakers can tailor regulations to minimize population-level harms of substance use. In many cases, policies that support harm reduction will be most responsive to managing the risks of substance use in light of differing levels of risk across a population.

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