New research continues to link marijuana with schizophrenia. Experts say it's not so simple

A study this week reported an association between people’s medical records listing cannabis use disorder and both schizophrenia diagnoses and psychosis — but experts emphasized the data should not be used to draw causal conclusions about cannabis use and these risks. 

In a study published in JAMA Network Open, the authors reported that the proportion of people with a new diagnosis of schizophrenia who had cannabis use disorder listed in their record increased after cannabis was legalized in Canada. The rates of nonspecified psychosis also increased during this time among people with this label in their record, said Dr. Daniel T. Myran, the lead author and a researcher at the Ottawa Hospital Research Institute.

“Seven percent of people in Canada use cannabis nearly every day, and there's now more people in the United States who use cannabis every day than drink alcohol every day,” Myran told Salon in a phone interview. “I see this as a signal that this is something that could actually become a pretty important public health concern.”

Across the study period, the overall incidence of schizophrenia remained stable, and it has remained stable since the 1990s. The authors state that this “occurred because the incidence of schizophrenia increased among younger individuals while decreasing in older adults" during the study period.

Yet this is important to note considering cannabis use has significantly increased in the past decade. 

“If cannabis use ‘causes’ schizophrenia, it is absolutely impossible that the rate of schizophrenia would not be going up with the rates of cannabis use increasing 1,000-fold,” said Dr. Peter Grinspoon, a cannabis specialist at Massachusetts General Hospital.

This isn’t the first time a link has been reported between cannabis and psychosis. The two have been tangled up together since the 1930s with the release of the anti-cannabis propaganda film, “Reefer Madness,” in which children used cannabis and experienced a series of hallucinations and negative side effects.

As researchers began examining whether there was a link between cannabis use and psychosis and schizophrenia, the nuances involved in many of these experiments were lost. A 2017 report from the National Academies of Sciences, Engineering, and Medicine concluded: “There is substantial evidence of a statistical association between cannabis use and the development of schizophrenia or other psychoses, with the highest risk among the most frequent users.”

But many assumed a causal relationship from studies like these, which were not designed to be able to determine causal relationships. An alternate hypothesis, for example, suggests the possibility that people who are prone to schizophrenia are using cannabis because it helps treat some of their symptoms.

Although some have pointed out that implying causation with this association is not possible with this kind of data, the former narrative has continued to be perpetuated in the media and scientific journals across decades.

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“Debates on the science of cannabis risks have always been highly politicized, characterized by subjective takes and often cherry-picked data to support different views — whether in supporting or opposing reforms like decriminalization or legalization,” said Steve Rolles, a senior policy analyst for the Transform Drug Policy Foundation. “This problem is exemplified by the debate on cannabis and psychotic illness risks, with people able to trawl the voluminous body of research, and then amplify or ignore the findings that either support or undermine their particular position.”

High-potency cannabis with THC levels above 15% can produce psychotic states that cause paranoia and hallucinations, but this is a temporary state that does not mean people will go on to be diagnosed with schizophrenia, said Dr. Muhammad Aadil, an addiction psychiatrist at the Albert Einstein Medical School in New York. However, researchers could not measure the type of cannabis and its potency in this study.

“People are just having a bad reaction because of two reasons in my opinion,” Aadil told Salon in a phone interview. “They don’t know what they are consuming and they don’t know what its effects are going to be on their body.”

Myran emphasized this was not a causal study and that there were several factors that could not be accounted for that could be confounding the results, like whether patients had a family history of mental health disorders.

In another study also published last week, his research group also reported an association between people who had been labeled as having a cannabis use disorder on their medical record and increased mortality. Similarly, many people might be using cannabis to self-medicate for underlying conditions that could explain some of those associations — in other words, sick people at risk of death may seek out marijuana — and some important potential confounding factors, like whether patients used tobacco, were not included.

Yet understanding the characteristics of these patients is crucial when making associations like this, especially because more people who were considered to have cannabis use disorder in the study had conditions like hypertension, asthma and cardiovascular disease, said Dr. Carl Hart, a researcher at Columbia University.

“That is not typically what you see in cannabis users,” Hart told Salon in a phone interview. “I don’t know who this population is, and it could be people who are just having problems, and who happen to smoke cannabis, too.”

In both studies, people were listed as having a cannabis use disorder based on whether they visited the emergency department for care related to cannabis use. However, cannabis use disorder is a diagnosis that typically requires a more detailed evaluation by a health care provider than what might be performed in an emergency department, Hart said.

That means some patients might have been classified as having cannabis use disorder when they did not, which the authors acknowledged in the study.

“A lot of people took too big of a gummy and had anxiety and went to the hospital, and they’re calling that cannabis use disorder,” Grinspoon told Salon in a phone interview. “If you just go by hospital spreadsheets it's completely misleading.”

Research on cannabis is further behind many other areas of study because it is still federally recognized as a controlled substance. And although federal funding has increased over the years for these studies, they tend to focus more on potential harms than on cannabis’ therapeutic potential.

Although the Biden administration proposed moving cannabis from a Schedule I substance to Schedule III, which would remove most criminal penalties and barriers to conducting research, it has not been enacted yet. It’s unclear whether the Trump administration will continue those efforts.

Regardless of whether cannabis is legal or not, people will continue to use it, and educating them about safe use has been shown to be the most effective way to reduce harm.

“Criminalization and prohibition makes doing this much more difficult,” Rolles said. “Resources are siphoned away from proven public health measures into counterproductive enforcement, and the stigma of criminalization pushes key target populations away from the very services they could most benefit from.”