About one in five young adults in the United States uses marijuana, and that could be contributing to the high rates of depression and suicide in that group, a new study suggests.
The research, published Wednesday in JAMA Psychiatry, reviewed data from nearly a dozen studies that included more than 23,000 people. It found that marijuana use in adolescence is linked with an increased risk of depression, as well as suicidal thoughts and attempts, before age 32.
This doesn’t mean marijuana causes mental illness, the researchers cautioned. The studies included in the analysis looked only at associations between the two, not cause and effect.
“But if you put everything together — animal studies, brain imaging studies, these types of meta-analyses where you study association — we have an indication at least that adolescents should be aware not to smoke cannabis,” said Gabriella Gobbi, a coauthor of the study and professor of psychiatry at McGill University in Montreal.
For the review, researchers pooled data from 11 previous studies on marijuana and depression, anxiety, and suicide. They included two dozen more studies in the qualitative analysis.
Recognizing that youths with depression often self-medicate with marijuana, the researchers only included longitudinal studies in which children weren’t depressed or suicidal at the start. Longitudinal studies follow participants over time and collect data every few years. They’re considered reliable measures of change because of the consistent follow-up.
“Given these restrictive characteristics, we have confidence that the data is quite good,” Gobbi said. “Not perfect, but good.”
The researchers concluded that about 400,000 cases of adolescent depression in the U.S. are potentially attributable to marijuana. “Cannabis is a serious public health concern,” they wrote.
The National Institute of Mental Health estimates 3.1 million adolescents experience at least one major depressive episode per year.
The new study did not find a significant association between marijuana use and anxiety.
The findings come as marijuana legalization is spreading rapidly. Pennsylvania and New Jersey are among more than 30 states that have legalized medical marijuana. Ten states have legalized recreational use, too. (Canada legalized recreational marijuana nationwide in 2018.)
Sales to minors are prohibited even where the substance is legal, but national data show youths are still able to access it. About 14 percent of eighth graders and 32 percent of 10th graders have used marijuana, according to the National Institute on Drug Abuse. And the Centers for Disease Control and Prevention reports more American teens use marijuana than smoke cigarettes.
The drug could be affecting the development of their brains, Gobbi said. Tetrahydrocannabinol (THC), the main psychoactive component of marijuana, interacts with cannabinoid receptors in the brain. These receptors are located in areas that are involved in mood control.
Interfering with brain development might lead to an increased risk of mood disorders, such as depression, Gobbi said. And that in turn, might contribute to an increase in suicidal thoughts and attempts.
But much more research is needed, Gobbi said.
Little is known about how dose or frequency of marijuana use affects adolescent mental health, if there are differences between girls’ and boys’ responses to the drug, or if there is a particular age at which children are most vulnerable.