Teens Beware: Using Marijuana Could Make You Dumber

Photo: Chuck Grimmett/Flickr

Marijuana is the world’s most widely used illicit drug. But smoke too many joints too soon, and you might lose IQ points: A new study suggests extensive marijuana use starting as a teenager could lead to cognitive decline.

In a study of 1,037 New Zealanders followed from birth to age 38, people who began using marijuana as adolescents and used it extensively for years saw their IQs drop by about eight points. What’s more, among adolescent-onset users, quitting the drug did not reverse the mental deficits.

“Marijuana is not harmless, and particularly not for adolescents,” said study author Madeline Meier, a psychologist at Duke University. Meier’s findings were reported Aug. 27 in the Proceedings of the National Academy of Sciences.

Researchers gave study participants an IQ test at age 13, before the start of marijuana use, and again at age 38, after some had developed “cannabis dependence” — defined as continued use of the drug in spite of major health, social, and/or legal problems from using it.

To get a sense of how significant a decline of eight IQ points is, consider this: Having an average IQ of 100 puts you in the 50th percentile for intelligence, whereas an IQ of 92 slides you down to the 29th percentile.

In addition to showing IQ decline, stoners performed worse than their counterparts on tests of five specific areas of mental function, including memory, processing speed, and executive functioning. When surveyed, family members and relatives of participants also reported noticing more attention and memory problems among those who used marijuana extensively.

During adolescence, the brain undergoes extensive development, and heavy marijuana use might be interfering with this mental maturation. Although previous studies have linked heavy marijuana use to cognitive impairment, they couldn’t prove whether marijuana use caused the impairment, or the impairment caused the marijuana use. ”It was a chicken-or-egg problem,” said Meier.

“This study’s unique in that they’ve got very good assessment of people’s cognitive abilities before marijuana use,” said Australian psychologist and epidemiologist Wayne Hall of The University of Queensland. Others in the field agree. “It certainly confirms our earlier studies,” said cognitive scientist Staci Gruber of Harvard University, whose research suggests that starting to smoke marijuana before age 16 is more likely to cause cognitive impairment than starting later. Neither Hall nor Gruber were involved in Meier’s study.

The new study has its limitations. The researchers relied on subjective reports of marijuana use rather than actual chemical testing. While they controlled for certain factors which could skew the results, such as other drug addictions and education history, other factors could have contributed to the IQ decline.

“Despite the strength of this longitudinal design, it is not possible to definitively prove causality — that the cognitive decline was specific to exposure to cannabis and not some other unmeasured confound,” Australian psychologist Nadia Solowij of the University of Wollongong wrote in an email, though like Hall and Gruber, she found the study convincing.

Up for debate is whether the brains of adolescent-onset users recover after quitting marijuana use. Meier’s finding that quitting did not restore mental function to pre-use levels was based on only a small sample of subjects who quit.

Other studies suggest there is some recovery. Harrison Pope of Harvard Medical School and colleagues found that users who showed cognitive deficits a week after quitting had recovered fully after 28 days of abstinence.

Nevertheless, ”It’s becoming fairly clear that marijuana does not appear to be a benign substance for regular users starting at an earlier age,” Gruber said. But even if we accept that adolescent-onset marijuana use causes cognitive decline, the obvious questions are, how young is too young, and how much is too much?

“I’m not going to say that adult-onset cannabis is safe,” said Meier. “But I would tell parents to talk to their kids and tell them to delay their onset until adulthood.”

Citations: “Persistent cannabis users show neuropsychological decline from childhood to midlife.” By Madeline H. Meier, Avshalom Caspi, Antony Ambler, HonaLee Harrington, Renate Houts, Richard S. E. Keefe, Kay McDonald, Aimee Ward, Richie Poulton, and Terrie E. Mofitt. Proceedings of the National Academy of Sciences, August 27, 2012.

“Age of onset of marijuana use and executive function.” By Staci Gruber, Kelly Sagar, Mary Kathryn Dahlgren, Megan Racine, and Scott Lukas. Psychology of Addictive Behaviors, November 21, 2011.

“Neuropsychological performance in long-term cannabis users.” By Harrison G. Pope, Jr, Amanda J. Gruber, James I. Hudson, Marilyn A. Huestis, and Deborah Yurgelun-Todd. Archives of General Psychiatry, October, 2001.