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Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The Claim: Smoking marijuana does not make teenagers stupid, concludes a study in the current Proceedings of the National Academy of Sciences, contrary to other research that linked getting stoned to impaired cognitive function in adolescents.

The Backstory: Research on how marijuana affects the developing brain goes back decades, but has heated up as more US states decriminalize or legalize the drug for adults. According to 2015 government data, 15 percent of 10th graders and 21 percent of 12th graders have used marijuana in the past month.

Most studies have compared marijuana users to non-users at a single point in time, and so can’t assess how marijuana changes brain function over the years; any observed differences might have always existed, marijuana or not. And all the studies have been observational — assigning some people to smoke marijuana and others to abstain would yield cleaner results but is obviously unethical. Given those limitations, it almost doesn’t matter what the studies have found, but here’s a sampling: marijuana use is associated with decreased intelligence, a 2007 study from New Zealand found; or with poorer memory, according to a 1996 study of Costa Rican men and a 2002 US study of long-term users; or with poorer attention and verbal skills, a 2010 study reported. Other studies, but fewer, find no long-term association between marijuana and IQ, like this one from Canada in 2005 and this 2004 research on twins by scientists at Boston University.


In a key paper from 2012 — it was the largest and longest-running study to date, testing 1,037 people at age 13 and 38 — scientists at Duke University found a dramatic drop in intelligence among long-term marijuana users “suggestive of a neurotoxic effect of cannabis on the adolescent brain,” they wrote in PNAS.

As is often the case, even reputable researchers publishing in reputable journals run roughshod over the difference between correlation (two phenomena go together) and causation (one causes the other). One 2015 paper declared that “regular cannabis use in adolescence approximately doubles the risks of … cognitive impairment.” No: At most, such use is associated with impairment. Observational research cannot rule out the possibility that a third factor causes both cognitive decline and marijuana use.


First Take: The new PNAS study is also observational, but with an important twist: It zeroes in on that “third factor” possibility by studying twins, 789 in California and 2,277 in Minnesota. The twins underwent standard intelligence tests at ages 9 to 12 and again at 17 to 20, and reported whether or not they smoked marijuana (60 percent in California and 36 percent in Minnesota did). In hundreds of cases, one twin smoked and the other didn’t.

Overall, users’ IQ dropped 4 points relative to non-users over the study period. But more frequent marijuana use wasn’t associated with greater IQ decline, as you’d expect if marijuana were toxic to brain function. And measures of so-called inherent intelligence, like problem-solving, didn’t fall in users; on some measures, like puzzle-solving, scores actually rose. That also undermines the idea that marijuana impairs cognition. Instead, said co-author Joshua Isen of the University of Minnesota, it suggests that something was going on “in how much information they were absorbing,” which can reflect truancy, conscientiousness, and other factors apart from inherent cognitive ability and brain function.

The strongest evidence against the idea that marijuana is neurotoxic was that marijuana-using twins showed no greater IQ decline than their non-using siblings. “This fails to support the implication of the [2012] study that adolescent use of marijuana causes neurocognitive declines,” the researchers wrote in PNAS. Instead, it suggests that any drop in brain function is not from the drug but from “factors that underlie both marijuana initiation and low intellectual attainment” and are present in both twins’ lives.

That is, whatever spurred teenagers to start using also hurt their IQ.

Second Take: Although the findings challenge the idea that marijuana use hurts cognitive ability, it’s not the final word. If the drug has only a minor effect, this study might have been too small to find it. And since the researchers didn’t measure working memory or executive function (judgment, decision-making ability, and other higher cognitive skills), they couldn’t rule out marijuana’s effect on those. Also, some twins dropped out of the research; if they were greater users than those who stayed, the study would have missed a possible effect of heavy marijuana use.

The Takeaway: This study can’t rule out the possibility that marijuana hurts teenagers’ brains, especially in ways that show up only after decades. But it undermines claims that it does.

  • People who smoke weed and are achievers usually are already genetically gifted and would be successful even if they were deadbeat drunks. Weed maybe impairs their best potential, but that’s it.
    These people are NOT good examples of weed effects.

    Average person smoking weed only makes himself dumber, slower, clusters up reasoning and hurts his potential in every imaginable way.
    Trying weed probably has no effect on a person.
    Doing in regularly probably leaves a seriously harmful effect.

    Note: I smoked quite a lot of weed in late adolescence (about years 26-30) and I definitely got depressed much more often, lost some ability to form memories coherently. There was no external reasons for me to deteriorate, as I always have kept healthy and fit lifestyle.
    Only abstaining from weed (yes, even from 1x/week “light” smoke) for over 6 months started to make me see world much brighter again, made me enthusiastic to work more.

    • nah you’re just ignorant. Sometimes people have dark times in their life and during those times being high is better than being sober. While you’re high, time isn’t real and you can finally relax. If you can get by without it good for you, but just because someone is in a dark place and decides to do drugs, doesn’t mean they’re dumb. Sometimes you just really can’t see the light at the end of that dark tunnel, and being high makes it a little bit better even if it’s just for an hour in the day.

    • I have a high IQ. Scored over 87%, up to 96% on all 5 GED test in high school after I starting using pot. Also got the highest test score in senior physics class, as the only junior, after ditching class for the first month, (just started smoking that summer). After an injury at Boeing, I went back to Jr college to learn HVAC engineering. A counselor there told me I had the highest verbal comprehension score she ever saw. Came out with the highest grade in all 6 classes. And just took a online IQ test last year and came out about the same as what my parents told me it was back in the 60’s. About 112 or 116 if I remember right. Been smoking a lot of weed for 49yrs now. No problems at all except the stupid laws by stupid fearful bigots.
      POT DOES NOT MAKE ANYONE STUPID, but a lot of stupid people smoke pot too, and they like to blame others, so they will always blame the drugs or whatever, so it my be confusing to someone trying to study this. Accidents are caused by stupid people or people being stupid, not drugs or alcohol. The same as guns don’t kill people, stupid people kill people. It is the stupid factor that needs the punishment. The drugs, phones, or alcohol or whatever is irrelevant.
      Weeds main thing is it changes ones attitude a little to be able to tolerate crappy things that you have no power to change like traffic, a bad boss or job, or just life in general. But the bad effect is the same attitude adjustment can let you tolerate bad things that you do have the power to change and should do something about. As in “divorce that crazy girl, why are you still with her?”

  • It is not natural or normal to put any smoke in your lungs no matter the age. Thank you firefighters for saving people from smoke inhalation. Pot smoking and cigarettes are just a slower death than dying quickly in a fired up smoke filled room.

    • “Life’s a bitch and then you die, that’s why I get high cuz you’ll never go when you’re gonna go.” Life is too short to be so self conscious.. people who smoke can live over 100… even the tobacco smokers… weed never killed anyone physically. it doesn’t ruin your life, just don’t get addicted

    • Tobacco smoke and Marijuana smoke are two different types of smoke. THC is a much healthier chemical than Nicotine and the numerous carcinogens that go with Cigarettes. Sure, anything in your lungs besides air can be harmful, but there has been little medical issues with long term weed users, opposed to cigarette smokers. Bottom line, Weed doesn’t kill people, and is non-addictive, unlike Cigarettes.

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