Using marijuana as a teen is associated with a higher chance of experiencing depression as an adult, according to a new study that adds to a growing debate over the health effects of marijuana.
Cannabis is the most popular drug for teens, who are increasingly turning to marijuana before alcohol and tobacco. Yet even as support for marijuana legalization grows, there has also been a push to highlight the downside and potential dangers of marijuana and mental health (including a much-hyped but poorly reported new book). For today’s study, published in the journal JAMA Psychiatry, researchers from McGill and Oxford Universities analyzed 11 studies that covered nearly 24,000 teens. They found that using marijuana at least weekly before 18 is associated with a 37 percent increased risk of depression in adulthood (up to age 32), even when taking into account someone’s existing mental health issues. In fact, the researchers estimate that about 400,000 cases of depression in the US could be associated with teen cannabis use.
The results also suggested that teens who used marijuana were three times more likely to attempt suicide, although that finding was not very statistically powerful. There was no significant finding when it came to cannabis and anxiety.
As always, it’s important to note that the results show associations and not causal links, and are still based on only 11 studies. The researchers screened over 4,000 papers before finding the few that specifically tracked marijuana use and mental health starting in adolescence. Additionally, the studies didn’t provide information on how much cannabis the teens smoked, or the potency. Considering that the subjects were teens decades ago, when marijuana was much less potent, that is one big omission.
“The results have to be interpreted with caution because it’s based on a very small number of published studies, but the signal is quite strong,” says Guohua Li, an epidemiologist at Columbia University’s Mailman School of Public Health who was not involved with the study. (The study authors were not available for interview as of press time.) “The implications for the study of public policy and mental health are profound,” he adds, because marijuana is so common, with about 20 percent of high school seniors in the US using the drug at least once a month.
The study gives voters and policymakers more information to consider, as marijuana legalization continues to spread in the US. Currently, 33 states have legalized medical marijuana while 10 have approved recreational marijuana, and many politicians have come out in favor of legalization. “It’s going to take much more research and learning years to understand the full picture of the impact of marijuana legalization and population health,” says Li. “I think protecting children and adolescents and monitoring these small effects is very important.”