Here's What We Know About What Weed Does to Teens

Just two days before the New Year, a University of Montréal study made headlines when it concluded that, compared to kids who start using marijuana at age 17, those who take it up at 14 are at greater risk for some learning and problem-solving deficits.

"There is no particular developmental event that happens at 17," says Natalie Castellanos-Ryan, the psychologist who led the study. "The brain has just simply had more time to develop, making the brain of a 17-year-old less vulnerable to the [drug's] neurotoxic effects."

Castellanos-Ryan makes a point of saying that her study—while it controlled for "a whole range" of confounding factors, including behavioral issues and other types of substance abuse—does not prove a cause-and-effect relationship between pot and impaired brain development. But it adds to a growing pile of evidence that links a steady marijuana habit to poorer mental and cognitive health outcomes among teens.

Nationwide, rates of teen drug use are falling. The percentages of high schoolers using meth, cocaine, heroin, and inhalants are at all-time lows, according to the National Institute on Drug Abuse's latest "Monitoring the Future Survey." While marijuana use isn't bottoming out, fewer 8th and 10th graders are smoking weed now than did five years ago, even as states have relaxed laws surrounding marijuana possession and use.

Still, marijuana remains the most commonly used illicit drug among kids. Roughly one in three high schoolers smokes pot, the NIDA data shows. Not surprisingly, marijuana use peaks among 12th graders, 6 percent of whom use daily. But the question of whether smoking weed damages a teen's brain is a difficult one to answer conclusively.

"I take umbrage when I see scientists saying there's a causal relationship between marijuana and negative brain outcomes," says Margaret Haney, director of the Marijuana Research Laboratory at Columbia University.

That doesn't mean Haney believes marijuana is safe for teens. She says most of the data we have suggests exactly the opposite. "We have these clear associations with people who start smoking heavily in adolescence and go on to have worse outcomes in several areas," she says. She mentions links between heavy marijuana use and poor academic performance, as well as mood, psychosis, and cognitive deficits.

But proving that marijuana is the cause of these brain issues, and not just something troubled or challenged teens are more likely to take up is very difficult. "We see these associations over and over again, but there's no ethical way to do a randomized clinical trial on this, so we can't really pin them on marijuana," she says.

That said, there is a large nationwide effort underway to study the ways different substances and lifestyle factors—marijuana, but also habits like video-game playing and social media use—affect a kid's brain. Called the Adolescent Brain Cognitive Development (ABCD) Study, the project will involve more than a dozen major universities, and will follow roughly 10,000 kids from the ages of nine or ten all the way to young adulthood.

Deanna Barch, chair of the department of psychological and brain sciences at Washington University in St. Louis, is one of the ABCD Study's principal investigators. While she says the study is still in its "data collection" period, and so can't yet contribute to the conversation on marijuana and teen health, there are reasons to suspect a young person's brain may be more vulnerable than a grown-up's to the chemicals in marijuana. "There are really important developmental experiences and processes that are occurring in the teen years compared to the adult years," Barch says. These experiences and processes, she says, may make marijuana use "particularly risky" for kids.

Haney gets more specific. "During adolescence, there is rapid brain growth in the prefrontal cortex," she says. This region of the brain seems to play a role in tasks like decision making and social information processing, and also goal achievement and some aspects of personality. It also happens to be densely packed with cannabinoid receptors, Haney says.

These receptors respond to a specific class of chemical your body naturally produces. But your cannabinoid receptors also react to substances in marijuana, namely THC, that closely resemble the cannabinoid chemicals your body makes. (Cannabis and cannabinoids have the same word root because your cannabinoid system was named after the plant.)

Researchers are still sorting out the exact functions of your brain and body's cannabinoid system. But the interactions between it and marijuana help explain why smoking pot gets you high, and why it helps quell nausea among some patient populations. Because your prefrontal cortex is the last part of your brain to mature, and also because it's loaded with these receptors that are uniquely susceptible to the chemicals in marijuana, Haney says there's "biological plausibility" that lots of exposure to weed could mess with a kid's brain development. "But again," she says, "demonstrating causality is tough."

So yeah, we don't have studies showing causation. But what correlations have researchers turned up? Attention, learning, and memory all seem to suffer among teens who regularly use pot, concludes a 2007 study from New Zealand's University of Otago. In particular, regular marijuana users seemed to lag behind their non-using peers when it comes to "executive functions," including the ability to call up stored information in order to work out a complex problem, the study shows.

There's more research to suggest some of these info storage and recall issues resolve themselves when teens stop smoking for a few weeks. But the attention deficits—the ability to stay focused and on-task—may not fade so quickly, that study found.

There's also some evidence that marijuana could "disrupt normal [teen] brain developmental trajectories," according to a 2015 review from the University of California, San Diego. The authors of that review mention a 2010 study that found the earlier a teen starts smoking weed, the less volume he was likely to have in his prefrontal cortex—that region Haney described above. On the other hand, a 2011 study found that teen marijuana users have larger amygdalas—a part of the brain that handles emotional processing—and may be more likely to suffer anxiety or depression as a result.

But again, all of these findings are proof only of correlation. They're not hard evidence that smoking weed causes these changes in brain development or morphology. Also unclear at this point: how much weed a kid would have to smoke to increase his theoretical risks, or whether smoking low-THC strains could somehow ameliorate pot's harmful effects.

A lot of these questions may never be answered. Just as no doctor can tell you how many cigarettes you can smoke before developing cancer, any hypothetical cause-and-effect harms of marijuana among teens will always have an element of guesswork. But at this point, the evidence we do have suggests marijuana and a developing brain don't go well together. Until we know more, the less a teen uses, and the later they wait to start, the better.

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