Marijuana linked to mental health risks in young adults, growing evidence shows

New research, involving millions of people worldwide over decades, adds to worries that heavy use of high-potency cannabis could exacerbate the mental health crisis in the U.S.

“The ideal time to consider using weed — if you’re going to use it — is 26 or later,” said Dr. Ryan Sultan, a psychiatrist at Columbia Irving Medical Center. Kate Dehler for NBC News

New research published this month, involving millions of people worldwide over decades, is adding to worries that heavy use of high-potency cannabis and legalization of recreational weed in many U.S. states could exacerbate the nation’s mental health crisis in young adults.

“There is a big sense of urgency not just because more people are smoking marijuana, but because more people are using it in ways that are harmful, with higher and higher concentration of THC,” Dr. Nora Volkow, director of the National Institute on Drug Abuse (NIDA), said in an interview.

One of the studies, from researchers in Denmark in collaboration with the U.S. National Institutes of Health, found evidence of an association between cannabis use disorder and schizophrenia. The finding was most striking in young men ages 21-30, but was also seen in women of the same age.

The paper, published in the journal Psychological Medicine, looked at data from almost 7 million men and women in Denmark over the course of a few decades to look for a link between schizophrenia and cannabis use disorder.

The magnitude of the connection between cannabis and schizophrenia for young men surprised study author Volkow, who was expecting the number to be closer to 10%.

“This is worrisome,” she said.

There are now 22 states that allow recreational use of marijuana, with Minnesota likely to become the next state to legalize it.

Whether recreational cannabis laws contribute to underage consumption is unclear, but Volkow has made addressing cannabis use among teenagers one of NIDA’s top priorities. Daily marijuana use among young adults has risen to record highs, with more than 1 in 10 of young adults ages 19-30 now reporting daily use, and almost half reporting use within the last year, according to the agency’s most recent data.

Another study, led by Sultan and Columbia researchers published earlier this month, found that teenagers who use cannabis only recreationally are two to four times more likely to develop psychiatric disorders, including depression and suicidality, than teenagers who don’t use cannabis at all.

Because research to date has been observational and doesn’t directly prove cause and effect, the connection between marijuana and psychiatric disorders is controversial. It’s unclear whether people who already have or are developing psychiatric conditions are more likely to turn to cannabis as a way to self-medicate or whether cannabis use triggers mental problems.

Volkow is optimistic that a large ongoing study on adolescent brain development at the National Institutes of Health can help answer this question.

Sultan acknowledged the limitations of the evidence. “It’s sort of this circular feedback where they’re kind of just feeding off each other,” he said.

Dr. Deepak D’Souza, a psychiatrist at Yale University who has been studying cannabis for 20 years, insists there are too many lines of evidence to ignore.

“We may be grossly underestimating the potential risks associated with cannabis,” he said.

Given increasing legalization and rising potency in cannabis products, D’Souza has never been more worried about the mental health effects of cannabis use among youth.

“This is a massive concern,” he said. “We have been woefully inept in educating the public and influencing policy.”

Is legalization affecting rates of marijuana use?

Early data suggests that in young adults ages 18-25, legalization is leading to higher rates of cannabis use, particularly in Oregon and Washington, according to an analysis published earlier this month in the journal Substance Abuse.

The research, led by researchers from McMaster University in Canada, found the evidence in other age groups a little less clear, and more research is needed to understand how legalization is affecting rates of cannabis use.

In areas where marijuana becomes legal and easier to access, Volkow’s concern is the ease with which products can be mixed, leading to a high total dose of marijuana consumed.

One of the biggest issues, she says, is the lack of regulation on the concentration of THC in products.

Marijuana consumed decades ago had concentrations of THC, the main psychoactive ingredient, of 2 to 3%, but cannabis products today can have THC levels as high as 90%.

“That’s not even the case for alcohol as you cannot put more than a certain percent alcohol into liquor,” she said. “The same thing with tobacco cigarettes, you regulate how much nicotine they have. Here, we have no regulation.”

THC potency is significant, Volkow said, because cannabis is more likely to be linked to psychosis with higher doses consumed.

What age is the most vulnerable?

Research has shown that the human brain is the last organ to fully develop and doesn’t finish until the mid-to-late 20s. That makes adolescents and young adults particularly vulnerable to the effects of cannabis as their brains continue to mature.

“Really, the ideal time to consider using weed — if you’re going to use it — is 26 or later,” Sultan said.

People who wait until at least age 26 are much less likely to become addicted or develop mental disorders, said Dr. Sharon Levy, a pediatrician and addiction specialist at Boston Children’s Hospital.

“The greatest risks are clearly in the adolescent and young adult age range,” she said.

However, people with a family history of a psychotic disorder shouldn’t use cannabis at all, Sultan cautioned.

What does cannabis do to the brain?

Although scientists are still learning about the effects of marijuana on developing brains, studies so far suggest marijuana use in teenagers may affect functions such as attention, memory and learning, multiple studies have found.

“It’s somehow interfering with the connections that we use in our brain to distinguish between what’s going on in our heads and what’s going on outside of our heads,” Levy said in reference to the psychotic symptoms that can happen.

D’Souza added that cannabis use can have serious impacts on the developing brain because of its effects on the endocannabinoid system, a complex signaling system in the brain that marijuana targets.

“Endocannabinoid systems play an important role in sculpting the brain during adolescence, which is when schizophrenia usually manifests itself,” he said.

Disturbing that system with cannabis use could have “far reaching complex implications on brain development.”