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© Stockbyte via Getty Images / Illustration by Emily LankiewiczScientists are currently studying whether cannabis use may cause schizophrenia.
Recent studies have examined the relationship between the drug and psychoses and looked at how the brain responds to the substance

While cannabis has become a mainstay in recreational dispensaries and medical clinics, scientists are still learning about its long-term psychological impacts. The drug's increasing popularity โ€” both recreationally and to treat conditions like chronic pain and anxiety โ€” is making some researchers and consumers concerned over one of its most severe side effects: psychosis.

Cannabis is psychoactive, meaning it affects how a user's brain works. For decades, researchers have been identifying how cannabis' brain-affecting properties can cause symptoms of psychosis. Psychosis refers to any mental state where a person struggles to distinguish between what is real and what is not real. Psychosis is characterized by hallucinations, where people sense things that are not there, and delusions, where people hold inexplicable false beliefs. Some psychoses, like schizophrenia, are chronic, but psychosis can be drug-induced, with cannabis being considered one of the riskiest โ€” if not the riskiest โ€” drugs for chronic psychosis.

"Someone with cannabis-induced psychosis is at a higher risk of transitioning or being subsequently diagnosed with schizophrenia," says Daniel Myran, a public health physician at the University of Ottawa, "than someone with methamphetamine-induced psychosis, or, you know, who's having psychosis related to alcohol use or opioid use or cocaine use."

Myran is the author of a February report published in JAMA Network Open that looked at rates of psychotic disorders in the Canadian province of Ontario among people who were flagged for cannabis use disorder โ€” dependence on or abuse of cannabis, sometimes categorized as cannabis addiction. The study used data collected between 2006 and 2022 from over 13 million people in Ontario and found an association between cannabis use and psychosis onset.

The study looked at two types of psychosis, schizophrenia spectrum disorder and psychosis that did not fit neatly into a diagnostic category. Schizophrenia spectrum disorder includes many disorders characterized by hallucinations, delusions and a struggle to function. The other diagnostic category labels people who may have similar symptoms but do not meet all the criteria for a schizophrenia diagnosis. Researchers found that, while rates of schizophrenia stayed stable over time, the proportion of schizophrenia associated with cannabis use nearly tripled โ€” from 3.7 percent to 10.3 percent โ€” after the drug was legalized for nonmedical use across Canada in 2018. Rates of psychosis that didn't neatly fit into a diagnostic category almost doubled after legalization, with significant upticks among younger people. While the uptick occurred after cannabis legalization, Myran thinks this increase in non-schizophrenia diagnosis could also be affected by other drugs or mental health problems.

Myran's study is also limited by how it defined cannabis addiction. The study identified individuals as having cannabis use disorder when they visited the emergency room or were hospitalized for mental and behavioral disorders due to cannabis use or for cannabis poisoning.

Peter Grinspoon, a cannabis specialist at Massachusetts General Hospital who was not affiliated with the study, says that criteria could include people who do not have cannabis addiction, like someone who tries cannabis for the first time and panics or even hallucinates.

"They diagnosed a person who shows up with an overdose โ€” anybody could have an overdose," Grinspoon says. "You take a gummy the wrong size, you have an overdose." This overdiagnosis problem is not specific to Myran's study, he says. Across the board, Grinspoon thinks the way the medical community defines cannabis addiction is "broken."

To be diagnosed with cannabis use disorder, under the latest edition of the Diagnostic and Statistical Manual of Mental Disorder, a patient has to meet at least 2 of 11 criteria over a year, with two of those criteria being symptoms of withdrawal and tolerance. Since withdrawal and tolerance symptoms can occur in people taking cannabis according to a doctor's prescription, Grinspoon thinks these criteria could prompt "vast" overdiagnosis of cannabis addiction.

"We don't use those for medical opiates or medical benzodiazepines, because everybody that uses them has tolerance and withdrawal," says Grinspoon. "That's not addiction."

Despite this criticism, which Grinspoon says reflects an overdiagnosis problem across the field, he agrees that some kind of link exists between cannabis and psychosis. But he says further, more detailed research needs to be conducted, especially on causality.

An April study in JAMA Psychiatry may help identify where in the brain the proposed cannabis-psychosis link occurs. Jessica Ahrens, a neuroscientist studying for her PhD at Canada's McGill University, led the research. Ahrens and other researchers used magnetic resonance brain imaging to look at and around the substantia nigra, a region of the brain critical for the production of dopamine, the "feel-good" molecule that helps drive the brain's reward system. People with schizophrenia tend to produce more dopamine in the substantia nigra, and when that dopamine breaks down those patients produce a pigment called neuromelanin. Ahrens and her team looked specifically at neuromelanin.

While psychosis can have environmental and genetic causes, Lena Palaniyappan, a psychiatrist at McGill who worked on the study, says dopamine is where all those factors connect.

To see whether cannabis triggered the dopamine pathway similarly to psychosis, researchers took neuromelanin-sensitive brain images of people diagnosed with cannabis use disorder and saw elevated dopamine function, similar to that in psychosis patients. But because researchers are looking at neuromelanin, which can break down from chemicals other than dopamine, they need to find out if cannabis users have high concentrations because of dopamine or another chemical entirely.

Matthew Hill, a developmental neuroscientist at the University of Calgary, says that the study does not appear to offer a complete causal relationship between cannabis use and psychotic disorders, but it does confirm that cannabis sparks enhanced dopamine transmission in humans. But seeing whether the relationship is cannabis-specific is challenging, since a similar dopamine effect has been seen with other substances, including alcohol, Hill says.

So, what actually causes the link between cannabis and psychosis? Researchers have proposed three popular hypotheses: that something that causes chronic psychosis also draws people to cannabis; that people with early symptoms of psychosis are self-medicating with cannabis; and that, for some people, consuming cannabis actually causes psychosis. All three theories could be true, with certain cases having different causes.

"I think that there is a group of individuals who would have never developed schizophrenia ... if they had not regularly used high potency cannabis," Myran says. "I think the debate is more of: How large is that number?"

For now, researchers have no reliable way to differentiate between schizophrenia cases that were directly caused by cannabis and cases that involve confounding variables. For example, some people experiencing the first symptoms of a psychotic disorder, such as paranoia, unease and trouble thinking clearly, may self-medicate with cannabis to ease those symptoms. Meanwhile, people who had adverse childhood experiences tend to be at higher risk for schizophrenia, and they also tend to be a population that uses more cannabis. Researchers still do not know the extent to which any of these factors contributes to the link between schizophrenia and psychosis, says Grinspoon. "All we know is that there's an association."

While the nature of the link is still being researched, experts agree that cannabis is particularly dangerous for young people. Grinspoon advocates a "just say wait" mindset for young people interested in cannabis, choosing not to use it until they are older. Additionally, people with family histories of schizophrenia and psychotic disorder, as well as bipolar disorder, should stay away from the drug, especially as those disorders tend to develop during adolescence and late teens.

"There's a lot of cannabis use that happens during adolescence, and adolescence is also a period when a lot of psychiatric disorders, particularly schizophrenia and psychotic disorders, begin to emerge," says Hill. "So I don't doubt that there is a relationship there."

As with all drugs, the dose makes the poison. Research suggests that more potent cannabis, or cannabis with more of the psychoactive chemical THC, tends to be associated with worse mental side effects. According to Grinspoon, a relatively small dose can help with some anxiety symptoms and relieve nausea, while a large dose can cause paranoia and vomiting.

As cannabis becomes more widely legalized, experts also advocate for firmer regulations, with Myran saying that looking at tobacco regulations would be a good place to start. Grinspoon agrees with that starting point but believes that advertising for all drugs and alcohol should be banned.

"Cannabis advertising, along with alcohol and tobacco and pharmaceutical advertising, is really misleading and unhelpful," says Grinspoon.

While regulation could be helpful, experts also agree that criminalization is not the answer, especially as it disproportionately targets and punishes people of color. Researchers say that users should approach cannabis with caution and moderation. The drug can help relieve some side effects for cancer patients, while aggravating chronic pain in other people. And while new studies, like Myran's and Ahrens', may help solidify the link between cannabis and psychosis, researchers do not want to spark panic.