By Lauren Smith

Updated: 03 July 2023 & medically reviewed by Dr. Kimberly Langdon

Up to 30% of cases of schizophrenia among young men could be attributed to cannabis (marijuana) abuse, a population-level study conducted in Denmark has found, exposing the sometimes devastating psychological impact of a drug hurtling toward full legalization and acceptance in many countries.

Young Men at The Highest Risk of Developing Schizophrenia as a Result of Cannabis Use

Risk of cannabis-linked schizophrenia highest among men ages 21 to 30

The study, published in Psychological Medicine in May, used the nationwide health registers in Denmark to analyze the medical records of more than 6.9 million people who were aged 16 to 49 at some point between 1972 and 2021.[1] The records were examined for incidents of schizophrenia and cannabis use disorder (CUD), the continued use of the psychoactive substance despite psychological, physical, or social harm.[2]

While previous studies have outlined a link between daily or near-daily cannabis use and new schizophrenia diagnoses, the research is the first to examine the risk across different sexes and age groups at a population level.[3]

“This is the first time we’ve seen a large-scale study across an entire population that addresses the relationship of cannabis and schizophrenia across different age and sex groups,” Wilson M. Compton, deputy director of the National Institute on Drug Abuse (NIDA), which collaborated with the Mental Health Services in the Capital Region of Denmark on the study, told Scientific American.[4]

The study revealed that men, especially young men, are at the highest risk of developing schizophrenia linked to cannabis use.

The researchers estimate that 15% of cases of schizophrenia among men ages 16 to 49 in 2021 may have been averted by preventing cannabis use disorder, compared to just 4% of schizophrenia cases among women in the same age bracket. 

The risk of cannabis-induced schizophrenia is even higher among young men. Up to 30% of preventable schizophrenia cases among men aged 21 to 30—or 3,000 cases in total— could have been averted if the patients hadn’t developed cannabis use disorder, researchers theorized.

Risk has risen with the potency of cannabis

The study builds on previous evidence uncovered by the same researchers in 2021 that the fraction of schizophrenia cases attributed to cannabis abuse has risen precipitously over the previous decades. 

Using the same corpus of Danish health records, they found that the population-attributable risk fraction for cannabis use disorder in schizophrenia—the proportion of cases of a disease that can be attributed to a risk factor across a population—rose from around 2% in the decades before 1995 to 6% to 8% since 2010, a three- to four-fold increase.[5][6] 

That rise has corresponded with an increase in the potency of cannabis in Denmark, from 13% THC content in 2006 to 30% in 2016.[7]

“While [the study] isn’t proving causality [between high potency cannabis and schizophrenia], it’s showing that the numbers behave exactly the way they should, under the assumption of causality,” said Carsten Hjorthøj, lead author of both that study and the paper from May.

Increases in the potency of cannabis have come as the drug has been legalized in many Western countries, although it remains illegal except for medicinal use in Denmark. The researchers cautioned that the normalization of cannabis use in many countries and great interest in its therapeutic applications have obscured the risks of problematic use.

“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,” said Hjorthøj, associate professor at the Mental Health Services in the Capital Region of Denmark and at the University of Copenhagen. “This study adds to our growing understanding that cannabis use is not harmless and that risks are not fixed at one point in time.”[2]

Related blog: Germany Unveils Plan to Legalize Recreational Marijuana

Scientists unsure if risk differential is a result of brain differences or young men’s more frequent use of cannabis

Neither study illuminates exactly why cannabis abuse might cause schizophrenia and why young men in particular may be more vulnerable. Compton told Scientific American that future research would probe whether the elevated risk in young men compared to young women was due to a difference in adolescent male brains that makes psychosis more likely to be triggered by the substance or it’s simply down to men’s more frequent exposure to cannabis.[4]  

Previous studies into marijuana's effect on brain function have found that men use cannabis more commonly, frequently, and in higher volumes than women do and also begin using it at younger ages. Indeed, Being male is one of the greatest risk factors for developing cannabis use disorder.[8]

Findings could inform policymaking

Despite the uncertainties, this study highlights the urgency of treating cannabis use disorder and may inform both policymaking and the decisions individuals make about their own use of the substance.

“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 illnesses associated with cannabis use,” NIDA director Nora 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.” 

“People are their own agents,” Hjorthøj added. “They can decide for themselves. But they should, if they do use cannabis, decide based on proper data and not from a story that cannabis is completely harmless and maybe even something everybody should use, which I think is the way the public discourse is moving.”

Related blog: How Long Does Cannabis (THC) Stay in Your System?

However, some researchers have thrown doubt on the cause-and-effect relationship between cannabis and psychosis. They suggest instead that heavy cannabis use and schizophrenia may have shared risk factors and emerge independently in the same people. 

A 2016 review made that argument, arguing that research suggests people with a vulnerability to psychosis are more likely to engage in early and heavy cannabis use as they do other problem behaviors, including alcohol use, cigarette smoking, and poor school performance. Cannabis use could be considered a prodromal (early) sign of schizophrenia, alongside anxiety, depression, mood swings, anger, social withdrawal, and truancy.[9][10] 

However, Robin Murray, co-editor in chief of Psychological Medicine, in which the Danish study appeared, said that papers problems specific factors, such as age and gender, involved in the cannabis-psychosis link, more closely than previous research. It's therefore more compelling and suggests "casual effect is almost certain."

But “it is currently impossible to prove a 100 percent, definite causal link between any environmental factor and schizophrenia because we do not have an animal model of schizophrenia," he acknowledged.