Marijuana may be worse for teen brains than alcohol, study finds

Researchers found that cannabis affected cognitive health more than alcohol and that those negative effects endured even after teens stopped using pot.

by Shamard Charles, M.D.

Recreational marijuana use is becoming more common, with its legalization in nine states and the District of Columbia, but in a study published on Wednesday, researchers suggest proceeding with caution as science considers its effects on the adolescent brain.

Scientists at the University of Montreal who conducted research into how weed affects brain development over time found that regular marijuana users, especially those who begin using it earlier in their lives, were far more likely to suffer repercussions in their thinking ability.

The study, published in the American Journal of Psychiatry, followed over 3,800 adolescents from 31 Montreal-area schools over four years. The teens, who started participating in the study when they were 13, agreed to provide the team with annual reports of marijuana and alcohol use frequency and took computer-based cognitive tests that measured recall memory, perceptual reasoning, inhibition and short-term memory.

Students were assured that parents and teachers would not have access to the information unless their habits indicated an imminent risk to their lives. The study concluded that marijuana affected teenagers’ long-term cognitive abilities more than alcohol use. And even after students reported stopping cannabis use, their cognition did not improve.

This is not the first study suggesting that early cannabis use may harm cognitive abilities.

In a study published in JAMA Psychiatry in June, J. Cobb Scott, an assistant professor of psychiatry at the Perelman School of Medicine at the University of Pennsylvania, and his colleagues analyzed 69 studies involving younger cannabis users. They found that compared with non-users, those who used marijuana frequently were more likely to have slightly lower scores on tests of memory, learning new information, and higher-level thinking involving problem solving and processing information.

But more research is needed to determine how and why the brain is affected by early marijuana use, scientists say. Patricia Conrad, the lead author of the Canadian study and a professor of psychiatry at the University of Montreal, told NBC News. “This study focuses on the neuropsychological effects of cannabis. We think it’s important because it is linked to how someone functions in life.”

“Cannabis causes cognitive impairment and delayed cognitive development in adolescents. Our study showed that early marijuana use has a lasting effect on cognitive ability.

The longer-term social implications of marijuana use in teens are more well known.

In 2014, The Lancet Psychiatry famously reported that teens who smoke marijuana daily are 60 percent less likely to graduate from high school or college than those who never use — and seven times more likely to attempt suicide.

And a 2017 study led by Josiane Bourque and colleagues at the University of Montreal suggests that a link between frequent marijuana use in adolescence and psychotic symptoms may be largely caused by depression.

According to data provided by the National Institute on Drug Abuse for Teens, about 5.9 percent of 12th graders reported daily use of marijuana in 2017, up from 5.1 percent in 2007.

The American Academy of Pediatrics published a report last year opposing medical and recreational marijuana use for kids, emphasizing that it’s best not to mess with the developing brain amid increasingly lax laws and attitudes on pot use.

“There is still significant cognitive development in the teen years," said Conrad, "so I would recommend that parents tell their kids to delay or limit use if possible."