Health and Medicine

Teens Beware: Using Marijuana Could Make You Dumber
Tanya Lewis

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.

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1 reply »

  1. As the article later states on the site, these things are not as clearcut as they seem. Having said that, I actually kind of like marijuana being considered worse because I find a lot of marijuana advocates and their organizations annoying, especially when you consider that a lot of them make the argument something to the effect of let’s legalize marijuana so we can divert our resources to going after the other “nasty” drugs. There’s a part of me that would almost like to keep marijuana illegal while legalizing all the hard stuff if for no other reason than my dislike of these potheads or as the blogger TGGP put it in this comment which I love , “If I could legalize crack, heroin and meth while punishing tokers with hanging, I’d do it”

    That whole thing about eight IQ points being such a huge deal is a little misleading btw. The fact is that the majority of IQ scores, as in other statistical phenomena, are clustered around the mean, so obviously a few points above or below 100 the mean will make a big difference in the percentile. I should also point out here that while I don’t deny the legitimacy of IQ as some pc types do, I really think that there are other abilities that are just as important to success in life and it is possible to have a relatively high IQ and yet be totally incompetent in life.

    Lastly, I would tell teenagers that if you are considering doing stuff that might grant temporal pleasure yet have negative consequences down the road, I’m to the point where I would say just do it because contrary to that syrupy propaganda It usually does NOT get better (regardless of your sexual orientation)

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