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.
"This is very, very, very worrying," says psychiatrist Dr. Ryan Sultan at Columbia University, a cannabis researcher who wasn't involved in the new study published in the latest JAMA Health Forum.
Strong study design
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 Dr. Lynn Silver, a pediatrician and researcher at the Public Health Institute, and an author of the new study.
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.
"Those are the scarier conditions that we worry about," says Sultan.
Silver points out these 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," Silver says.
Depression and anxiety, too
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," says Silver, "and anxiety went up by about a quarter."
But 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," says Silver. "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."
Silver hopes these findings will make teens more cautious about using the drug, which is not as safe as people perceive it to be.
"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 Sultan. 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 suggests a causal link between cannabis use and later mental health diagnoses. Additional research is needed to understand the link fully.
"Playing with fire"
Sultan, the psychiatrist and researcher at Columbia University, says the study confirms what he's seeing in his clinic — more teens using cannabis who've developed new or worsening mental health symptoms.
"It is most common around anxiety and depression, but it's also showing up in more severe conditions like bipolar disorder and psychosis," he says.
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.
"When someone has a psychotic episode in the context of cannabis or a manic episode in the context of cannabis, clinicians are going to say, 'Please do not do that again because you're you're you're playing with fire,'" he says.
Because the more they use the drug, he says the more likely that their symptoms will worsen over time, making recovery harder.
"What we're worried about [is if] you sort of get stuck in psychosis, it gets harder and harder to pull the person back," says Sultan. "Psychosis and severe mood disorders, particularly bipolar disorder are like seizures in your brain. They're sort of neurotoxic to your brain, and so it seems to be associated with a more rapid deterioration of the brain."
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