Nearly One Third of Schizophrenia Cases in Young Men Linked to Heavy Marijuana Use, Study Finds

As many as 30 percent of schizophrenia cases among young men may be linked to heavy marijuana use, according to a new study that analyzed data from more than six million people in Denmark.

The study, published this month in Psychological Medicine, also found the proportion of new schizophrenia cases linked to cannabis use disorder has risen consistently since the 1970s, likely due to the increasing potency of the drug that many view as harmless.

The researchers analyzed five decades worth of data from more than 6.9 million people. The study was led by researchers at the National Institute on Drug Abuse and the Mental Health Services in the Capital Region of Denmark at the National Institutes of Health, according to NIH.

The study adds to the mounting evidence that heavy use of high-potency pot can be harmful for people’s mental health, particularly for young people. And it comes as several states have recently legalized recreational marijuana use or are about to.

Last month, Delaware became the 22^nd state to legalize recreational marijuana. Maryland also legalized the drug this year, and Missouri legalized it last year. Minnesota is also on the verge of legalizing recreational pot – the state’s house and senate have both passed legalization bills, and they are in the process of resolving differences in conference committee. Ohio may vote on legalization later this year, and Florida legalization advocates are close to gathering enough signatures to put a pro-recreational pot constitutional amendment on the 2024 ballot.

“Increases in the legalization of cannabis over the past few decades have made it one of the most frequently used psychoactive substances in the world, while also decreasing the public’s perception of its harm,” Carsten Hjorthøj, lead author of the study, said in a prepared statement. “This study adds to our growing understanding that cannabis use is not harmless, and that risks are not fixed at one point in time.”

While previous research has shown that early and frequent cannabis use is associated with an increased risk of developing schizophrenia, the new study looked at the relationship between cannabis use disorder and schizophrenia based on sex and age.

The researchers estimated that 15 percent of schizophrenia cases among men between the ages of 16 and 49 may have been avoided had the men not first developed cannabis use disorder. And for young men between the ages of 21 and 30, the researchers estimated that as many as 30 percent of schizophrenia cases may be linked to marijuana addiction.

While the researchers found strong evidence of an association between marijuana use and schizophrenia among both men and women, the association was stronger among young men, according to the study. Researchers estimated that four percent of schizophrenia cases among women 16 to 49 could likely be linked to heavy marijuana use.

Last month, National Review highlighted the story of Catherine Mayberry, a one-time honor student and varsity athlete in Minnesota who, according to her parents, was diagnosed with schizophrenia after she became addicted to marijuana as an older teen. Her parents, Trent and Jane Mayberry, described their daughter’s decent into psychosis – she struggled to speak, blasted music to quiet the voices in her head, and had friends who likely weren’t real. She eventually started using harder drugs, and died in October from an overdose of methamphetamines laced with fentanyl.

“I’m 100 percent certain that it came from cannabis,” Trent Mayberry told National Review of his daughter’s psychosis. “If she never used cannabis, there’s a very high likelihood she would not have had these types of symptoms.”

Dr. Robin MacGregor Murray, a professor of psychiatric research at King’s College London, told National Review last year that those working in Britain’s psychiatric services mostly agree that there is a relationship between cannabis and psychosis. And, he said, there is “definitely” a connection with higher-potency pot. “I think that’s why I didn’t see people who were psychotic following cannabis use when I was young,” he said.

The entanglement of substance abuse disorders and mental illness continues to be a major public health issue, Dr. Nora Volkow, the National Institute on Drug Abuse director and a coauthor of the study, said in a prepared statement.

“As access to potent cannabis products continues to expand, it is crucial that we also expand prevention, screening, and treatment for people who may experience mental illness associated with cannabis use,” Volkow said. “The findings from this study are one step in that direction and can help inform decisions that health care providers may make in caring for patients, as well as decisions that individuals may make about their own cannabis use.”