A new cohort study found that schizophrenia cases related to cannabis use disorder serious enough to warrant a person to seek emergency care increased after cannabis became easier to access in Canada.
The findings are also likely relevant to the US, first author Daniel Myran, MD told ConsumerAffairs.
However, the findings may not be relevant to people who have milder cannabis use disorder or who just use cannabis and don’t have the disorder, one expert unaffiliated with the study, David Gorelick, MD, said.
The study, published in JAMA Network Open, found that the proportion of schizophrenia cases linked with cannabis use disorder (CAD) increased by nearly three times during a period of significant liberalization of cannabis policy in Canada.
At the start of the study in 2006, 7% of new cases of schizophrenia had received care for a cannabis use disorder before their diagnosis, and that's now gone up to 17%, said Myran, a public health and preventive medicine physician at University of Ottawa.
“The most important finding is that a growing proportion of new cases of schizophrenia have received care for a cannabis use disorder before their diagnosis,” he told ConsumerAffairs, “and that after you adjust for differences in people who have cannabis use disorder and people in the general population, there continues to be this very strong association and a growing association between cannabis use disorder and new cases of schizophrenia.”
About ten percent of cannabis users will become addicted, according to Yale Medicine, and that is known as cannabis use disorder (CUD). One of the biggest predictors of developing it is using cannabis daily, Myran says. The rate also varies substantially by frequency of use, according to an expert.
It is known that using cannabis is linked to developing earlier onset schizophrenia and psychosis, authors wrote in the study. The study was conducted because even though there have been concerns that legalizing cannabis could eventually lead to more schizophrenia cases, there has not been much evidence to support that claim.
“Compelling evidence” shows that cannabis use has substantially increased over time in both the U.S. and Canada, Myran said.
“Legalization seems to have broadly contributed to that, where it made it increase faster than it would have otherwise,” he said.
There’s also been an increase in the potency of cannabis in North America.
So, the question researchers now need to answer is: “Long-term, has cannabis legalization resulted in an increased number of people who develop schizophrenia,” Myran said.
Further study is needed because that’s a tough question to answer, he said. While more people are having episodes of cannabis-induced psychosis, “it takes time for people to start using cannabis, to develop a cannabis use disorder, have an episode or symptoms of psychosis, and then actually get diagnosed with schizophrenia.”
“Moving forward, I think that there should be lots of interest in understanding whether or not the number of people who have schizophrenia increases in North America,” he said.
Study methods
The 17-year study examined the population-attributable risk fraction — or PARF — of schizophrenia associated with cannabis use disorder and how it has changed over time. PARF in this case refers to schizophrenia cases that could have been avoided if cannabis use patterns serious enough to necessitate care in the emergency room or hospital were eliminated.
Researchers measured changes in PARF for CAD-associated schizophrenia over three policy periods in Canada: prelegalization (2006-2015), liberalization of medical and nonmedical cannabis (2015-2018) and legalization of nonmedical cannabis (20018-2022) among 13,588,681 people ages 14 to 65 in Ontario. They also analyzed the diagnosis of psychosis not otherwise specified (NOS) as a secondary outcome. No participants had schizophrenia at the start of the study.
Findings revealed that the adjusted population-attributable portion of schizophrenia linked to cannabis use disorder almost tripled from 3.7% in the perlegalization period to 10.3% in the postlegalization period.
Researchers also found that schizophrenia cases may be particularly elevated among younger males, in whom an estimated 19.9% of incident schizophrenia cases were associated with cannabis use disorder by the end of the study.
This pattern is concerning, study authors wrote, because it “may be capturing important harms associated with increasing cannabis use, as youths are considered the most vulnerable to cannabis use.”
“This seems to be a well-designed study, showing a significant association between having a cannabis use disorder severe enough to prompt an emergency department visit or hospitalization, and developing schizophrenia or schizoaffective disorder or psychotic disorder not otherwise specified… however, the association as studied only applies to the more severe end of cannabis use disorder so may not be generalizable to people with mild cannabis use disorder or people who use cannabis without cannabis use disorder,” Dr. David Gorelick, MD, PhD told ConsumerAffairs.
Gorelick is a clinical professor of psychiatry at the University of Maryland School of Medicine and Editor-in-Chief of Journal of Cannabis Research.
The findings have other limitations, researchers note, one of which is that increases in PARF may have been related to better detection and documentation of cannabis use disorder over time.
Role of legalization
“It’s very, very difficult to disentangle how much legalization has contributed to rising cannabis use,” first author Myran said.
“We won't know the full effects of cannabis legalization in Canada and the United States until maybe five or ten years after it's happened, or potentially even longer,” he said.
But that shouldn’t stop the creation of policies in the meantime, Myran said.
There’s a “decent body of evidence” that says daily cannabis use leads to adverse health and social effects if not for everyone than at least for some people, Myran said.
Therefore, it’s worthwhile to think about some of the policies society could put in place to reduce the amount of people who have cannabis use disorder or use it daily, Myran explained.
In the US, cannabis use — for medical or recreational purposes — is legal in many states but not at the federal level. In Canada, cannabis use has been legal for medical and recreational purposes for adults ages 18 and up since 2018.