Teen Cannabis Use Linked to Increased Risk of Mental Health Disorders

As marijuana use among teens has grown in the past decade, researchers have been trying to better understand the health risks of the drug. Now, 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.

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.

They found that 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.

Now, 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 new study also found that the risk for more common conditions like depression and anxiety was also higher among cannabis users. But the link between cannabis use and depression and anxiety got weaker for teens who were older when they used cannabis.

Silver hopes these findings will make teens more cautious about using the drug, which is not as safe as people perceive it to be. The new study is well designed and gets at the chicken or the egg, order-of-operations question.

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.

But 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.

He notes that 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. Because the more they use the drug, he says the more likely that their symptoms will worsen over time, making recovery harder.