Young men who use potent marijuana frequently have an increased risk of developing schizophrenia, according to a new study of almost 7 million health records.

As many as 30% of cases of schizophrenia among men aged 21-to-30 could have been prevented had they avoided cannabis use disorder, according to the study published Thursday in Psychological Medicine. The condition, loosely defined as frequent use of the drug despite negative consequences, has been found to develop in around three in 10 who use marijuana, according to past research. The latest study, based on Danish health records, adds to growing research into cannabis and mental health outcomes in the U.S. and other countries.

“The entanglement of substance-use disorders and mental illnesses is a major public health issue, requiring urgent action and support for people who need it,” said co-author Nora Volkow, who is also the director of the National Institute on Drug Abuse.

While the Danish population doesn’t have as much diversity as the U.S. and genetics plays a role in schizophrenia risk, the findings should concern people of all ethnic backgrounds, Volkow said in an interview.

“The evidence is sufficiently strong to encourage caution,” Volkow said, particularly for those with any family history of schizophrenia or early symptoms of the mental disorder.

Schizophrenia, characterized as a loss of touch with reality and by symptoms like delusions and hearing voices, can be a lifelong condition that’s difficult to treat. Schizophrenia’s economic burden on the U.S. was $343.2 billion in 2019, more than double what it was in 2013, according to a study in the Clinical Journey of Psychiatry.

The study is the first to show the relationship between cannabis use disorder and schizophrenia in a large population. Previous research has shown links between the use of cannabis with high levels of its psychoactive compound THC and psychosis, the loss of contact with reality that’s a main symptom of schizophrenia. Using cannabis at an earlier age and more frequent use are also believed to heighten the risk.

While cannabis use disorder isn’t responsible for most schizophrenia cases in Denmark, the study found, it contributed to a growing number of them over the past five decades. There was a significant difference between women and men; in the 16-49 age bracket, the study estimated that 15% of schizophrenia cases in men could be averted if they avoided cannabis use disorder, but only 4% of cases would be impacted in women.

The finding comes as many U.S. cannabis companies say they are increasing the potency of their products to keep up with consumer demand. Some U.S. states and Germany have recently considered whether to cap the potency of marijuana.

Cannabis is widely perceived to be nonaddictive and seen as beneficial for some mental health issues. But recent studies have shown that teen rates of addiction to cannabis are about the same as those to prescription opioids, there are serious risks from secondhand smoke, use of the drug in pregnancy, and an increased risk of heart disease. Some companies see opportunities to treat cannabis use disorder.

The authors said the increasing number of schizophrenia cases that can be blamed on cannabis use disorder, which has increased over the past 50 years, is likely due to the increasing potency of cannabis, as well as more diagnoses of the condition.

Carsten Hjorthøj, the lead author of the study, said that legalization has brought about a change in attitude, as fewer people perceive the drug to be dangerous. “This study adds to our growing understanding that cannabis use is not harmless,” Hjorthøj said in a statement.