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Roughly half of Americans use marijuana at some point in their lives, and many start as teenagers. Although some studies suggest the drug could harm the maturing adolescent brain, the true risk is controversial. Now, in the first study of its kind, scientists have analyzed long-term marijuana use in teens, comparing IQ changes in twin siblings who either used or abstained from marijuana for 10 years. After taking environmental factors into account, the scientists found no measurable link between marijuana use and lower IQ.

"This is a very well-conducted study ... and a welcome addition to the literature," says Valerie Curran, a psychopharmacologist at the University College London. She and her colleagues reached "broadly the same conclusions" in a separate, nontwin study of more than 2000 British teenagers, published earlier this month in the Journal of Psychopharmacology, she says. But, warning that the study has important limitations, George Patton, a psychiatric epidemiologist at the University of Melbourne in Australia, adds that it in no way proves that marijuana—particularly heavy, or chronic use—is safe for teenagers.

Most studies that linked marijuana to cognitive deficits, such as memory loss and low IQ, looked at a single "snapshot" in time, says statistician Nicholas Jackson of the University of Southern California in Los Angeles, lead author of the new work. That makes it impossible to tell which came first: drug use or poor cognitive performance. "It's a classic chicken-egg scenario," he says.

To better probe whether marijuana erodes IQ or inflicts harm in other ways, scientists have started following large groups of teenage drug users over time. The first study to do so, in Dunedin, New Zealand, in 2012 reported significant declines in IQ between ages 13 and 38 in heavy users compared with those who used marijuana occasionally before age 18 or not at all. The paper "had a major effect on thinking about the risks of early heavy exposure to cannabis," says Patton, a co-author. Critics, however, pointed out that the study failed to rule out other potential explanations for the decline in IQ, such as a teen's family environment or whether they dropped out of school.

One "powerful" way to address such concerns is to study identical twins, who share genes and upbringing, Jackson says. In the new study, he and his colleagues looked at 789 pairs of adolescent twins from two ongoing studies—one from the Los Angeles, California, area and the other from Minnesota—who enrolled between the ages of 9 and 11. Over the course of 10 years, the team administered five intelligence tests and confidential surveys about marijuana use. They also asked about other drug use such as opioid painkillers, cocaine, and binge drinking.

Marijuana users lost about four IQ points over the course of the study. But their abstinent twin siblings showed a similar pattern of decline, suggesting that the loss of mental sharpness was due to something other than pot, Jackson says. "Our findings lead us to believe that this 'something else' is related to something about the shared environment of the twins, which would include home, school, and peers," he says.

In the new study, teens who reported daily marijuana use for 6 months or longer did not show any difference in how much their IQ changed, compared with teens who had tried pot fewer than 30 times. This is a "clear indication that cannabis is unlikely to be the cause of any IQ decline," says Claire Mokryz, a Ph.D. student in Curran's lab.

But others say the new study has flaws—most importantly, a lack of detail about how often and in what quantity the teens used marijuana. The Minnesota and Los Angeles groups used different surveys about drug use. The Los Angeles group's questions were far less thorough, Patton says. In surveys administered to that group, for example, participants were asked, "Have you ever tried marijuana?" If a 13-year-old respondent answered "yes" after taking just one puff, they could be considered a drug user for every subsequent measurement. "My sense is that this paper does not do enough to dismiss the concerns from [our] Dunedin study about the effects of early heavy cannabis use" in teenagers, Patton says.

Sarah Feldstein Ewing, a psychiatrist at Oregon Health & Science University in Portland, agrees. "While it is possible that the findings are absolutely accurate," she says, the study represents a "missed opportunity to get a truly fine-grained analysis" of the contribution of cannabis and other substances to IQ.

Although there is "emerging evidence" that marijuana does not erode IQ, "this does not mean that heavy use in adolescence is problem-free," Jackson says. Other aspects of daily functioning could be affected, he says, adding, "we desperately need more research on the effects that marijuana has on the brain."

The best way to study marijuana's cognitive effects would be to administer the drug to individuals and see how duration, frequency, and dose affect the brain, Jackson says. "Unfortunately these types of studies are nearly impossible due to federal restrictions," he says. For now, he says, "I'm mostly concerned about what's going on in the child's environment, that a 14-year-old is seeking refuge in drugs."