Study Links Teen Cannabis Use to Doubled Risk of Bipolar and Psychotic Disorders

A new longitudinal study finds that cannabis use among adolescents increases risks of being diagnosed with bipolar and psychotic disorders, as well as anxiety and depression, years later. The study was published in JAMA Health Forum.

Researchers analyzed health data on 460,000 teenagers in the Kaiser Permanente Health System in Northern California. The teens were followed until they were 25 years old. The data included annual screenings for substance use and any mental health diagnoses from the health records. Researchers excluded the adolescents who had symptoms of mental illnesses before using cannabis.

"We looked at kids using cannabis before they had any evidence of these psychiatric conditions and then followed them to understand if they were more likely or less likely to develop them," says a pediatrician and researcher at the Public Health Institute, and an author of the new study.

The teens who reported using cannabis in the past year were at a higher risk of being diagnosed with several mental health conditions a few years later, compared to teens who didn't use cannabis. Teens who reported using cannabis had twice the risk of developing two serious mental illnesses: bipolar, which manifests as alternating episodes of depression and mania, and psychotic disorders, such as schizophrenia which involve a break with reality.

Only a small fraction — nearly 4,000 — of all teens in the study were diagnosed with each of these two disorders. Both bipolar and psychotic disorders are among the most serious and disabling of mental illnesses.

The illnesses are expensive to treat and come at a high cost to society. The U.S. cannabis market is an industry with a value in the tens-of-billions — but the societal cost of schizophrenia has been calculated to be $350 billion a year. "And if we increase the number of people who develop that condition in a way that's preventable, that can wipe out the whole value of the cannabis market," the study author says.

The new study also found that the risk for more common conditions like depression and anxiety was also higher among cannabis users. "Depression alone went up by about a third," and "anxiety went up by about a quarter."

The link between cannabis use and depression and anxiety got weaker for teens who were older when they used cannabis. "Which really shows the sensitivity of the younger child's brain to the effects of cannabis," the study author says. "The brain is still developing. The effects of cannabis on the receptors in the brain seem to have a significant impact on their neurological development and the risk for these mental health disorders."

"With legalization, we've had a tremendous wave of this perception of cannabis as a safe, natural product to treat your stress with," she says. "That is simply not true."

The new study is well designed and gets at "the chicken or the egg, order-of-operations question," says a psychiatrist and cannabis researcher at Columbia University who wasn't involved in the study. There have been other past studies that have also found a link between cannabis use and mental health conditions, especially psychosis. But, those studies couldn't tell whether cannabis affected the likelihood of developing mental health symptoms or whether people with existing problems were more likely to use cannabis — perhaps to treat their symptoms.

By excluding teens who were already showing mental health symptoms, the new study points to a potential causal link between cannabis use and later mental health diagnoses. Additional research is needed to understand the link fully.

Mental health disorders are complex in origin. A host of risk factors, like genetics, environment, lifestyle and life experiences all play a role. And some young people are more at risk than others.

Meanwhile, data from the National Institute on Drug Abuse-backed national Monitoring the Future survey shows that use of cannabis within the past year declined from 15.8 percent to 7.6 percent for 8th grade students, 28.7 percent to 15.6 percent for 10th grade students, and 34.7 percent to 25.7 percent for 12th grade students, from 1995 to 2025. The Drug Enforcement Administration acknowledged this trend in an online quiz on its "Just Think Twice" platform, stating that "From 1995 to 2025, past-year cannabis use decreased among 8th, 10th, and 12th grade students."