Teen Marijuana Use Associated With Drop in Intelligence

Does marijuana use during adolescence cause people to get dumber?

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Smoking marijuana as a teenager could cause lasting damage to a person's intelligence and memory, according to a new study released Monday.

People who started smoking marijuana as teenagers and continued into adulthood showed an average IQ drop of 6 points between age 13 and age 38, according to the study, which followed more than 1,000 New Zealanders from their birth to age 38. People who said they were "dependent" on the drug—meaning it affected their personal or professional life-- showed the greatest decrease.

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"If we were to legalize marijuana, we'd have to do something to limit the access of it to minors, similar to what's been done with alcohol and tobacco," says Madeline Meier, a researcher at Duke University and lead author of the paper, which was published in Proceedings of the National Academy of Sciences.

Though previous studies have suggested that marijuana could impair memory or intelligence if used during adolescence, Meier's study is one of the first that followed the same group of people for such a long time. The study also considered other potential causes of IQ decline, such as nicotine and alcohol use, hard-drug use, and schizophrenia. "We ruled out six alternative explanations for the observed effects of persistent cannabis use on neuropsychological functioning," the authors wrote.

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People who do not use marijuana saw no IQ decline, and people who started smoking in adulthood "did not appear to experience IQ decline as a function of persistent cannabis use," according to the study.

Scientists aren't sure why marijuana use during adolescence seems to impact intelligence, but they believe it's because marijuana can impair the brain's development during adolescence.

"The brain is undergoing importance developmental changes during adolescence," Meier says. "And it seems like marijuana can disrupt these critical changes."

Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at jkoebler@usnews.com.