Pot Dependence in Adolescence is Linked to a Long-Term Drop in IQ

By Sophie Bushwick | August 29, 2012 10:33 am


Compared to some of the drugs out there, cannabis can seem relatively harmless. It doesn’t have the ruinous effects of methamphetamines or even substances like synthetic pot. But there has long been suspicion that heavy use might have long-term effects on IQ, for instance [pdf].

Factors that tend to accompany cannabis consumption, such as the use of other drugs and alcohol and, in adolescents, a tendency to skip class, have made it difficult to decisively pin a dip in IQ to marijuana use. To clear away the noise, the authors of a new study in the Proceedings of the National Academy of Sciences turned to the reams of data from the Dunedin Multidisciplinary Health and Development Study, and they’ve found that on average, by the time they reached age 38, heavy pot users diagnosed with cannabis dependence during adolescence suffered an 8-point drop in IQ.

In the decades-long Dunedin study, originally designed to test the efficacy of new neonatal intensive care units, 1,037 babies born in Dunedin, New Zealand, between 1972 and 1973 became some of the most observed people you’ve probably never heard of. Researchers assessed the children every few years until age 21, and continued to follow up at ages 26, 32, and 38; the next check-in will be between 2016 and 2018, at age 44. As the subjects grew up, they provided a wealth of information about human development and have become the basis of over 1,100 papers and reports.

With the Dunedin study data, researchers could compare subjects’ IQ scores from age 13, before they began using any drugs, with their scores at age 38, and they could also eliminate confounding factors like education level and other drug and alcohol use. They found that those who used pot heavily showed a significant decline in IQ, particularly if they began during adolescence. The most frequent users experienced the most damage. And cutting down on marijuana after adolescence did nothing to alleviate the decline.

While these results suggest that developing adolescent brains are particularly vulnerable to the effects of marijuana and can suffer permanent harm from smoking pot, the drop occurred only in users diagnosed with cannabis dependence, which indicates more than just a fondness for marijuana, points out epidemiology blogger Suzi Gage. Such frequent users are not only more rare than casual consumers, they also tend to have other problems that getting high may help them face, such as depression.

So this IQ drop finding may not be applicable to the majority of adolescent pot smokers. But the study nevertheless suggests that if cannabis does ever become a legal substance in the United States, perhaps its use, like alcohol’s, should be restricted to adults.

Image of joint via Shutterstock

  • http://www.clear-uk.org Peter Reynolds

    You have this story back to front.

    The report shows that cannabis can be harmful to children but this is not news. It just reinforces what we already know. The REAL NEWS is that Professor Terrie Moffitt, of the Institute of Psychiatry at King’s College London, has said, unequivocally, that: “…cannabis is safe for over-18 brains”.

    This is the REAL NEWS!

  • Isabel

    “perhaps its use, like alcohol’s, should be restricted to adults.”

    duh, who has ever suggested otherwise?!

    “and they’ve found that on average, by the time they reached age 38, heavy pot users diagnosed with cannabis dependence during adolescence suffered an 8-point drop in IQ.”

    Where do you get this, the biggest drop for that group was 6 pt, so how could the average be 8?

  • Sophie Bushwick

    The largest decline was found in individuals who had been diagnosed with cannabis dependence 3 times. Within this group, those who became cannabis dependent during adolescence suffered an 8-point drop, not a 6-point drop.

  • Jake

    I don’t have full access to the article and supporting information, but it makes me wonder how they excluded other social factors. To me it stands to reason that not only depression or other self-medicating of psychological disorders, but also social factors that accompany heavy and prolonged use of cannabis especially as an adolescent (e.g. neglect, violence, poor role models, etc.).

    Maybe I misunderstand, but it appears to me that the author is claiming a relationship that is more than just correlation?

  • Erik

    At the news they also clarified that the drop in IQ was a lot less for the people that did not smoke cannabis for a week.

  • Isabel

    “Within this group, those who became cannabis dependent during adolescence suffered an 8-point drop, not a 6-point drop.”

    That’s not what is shown in Table 1. But I see in Figure 2 they have broken down that 3+ group (why not just show the numbers break down in the table?), which has gotten VERY small by the way- a couple of dozen people. Annoying when the abstract at a glance implies it is a large study, when it may be technically, it turns out to be a rather small study of adolescent users.

  • Isabel

    “Maybe I misunderstand, but it appears to me that the author is claiming a relationship that is more than just correlation?”

    They supposedly corrected for confounding variables, but not the excellent ones you mention like depression and neglect. These adolescents are likely to also have poorer diets and have exercised less.

    “We ruled out six alternative explanations for the observed effects of persistent cannabis use on neuropsychological functioning, namely that these effects could be explained by (i ) past 24-h cannabis use, (ii ) past-week cannabis use, (iii) persistent tobacco dependence, (iv) persistent hard-drug dependence, (v) persistent alcohol dependence, and (vi) schizophrenia. “

  • Brian Too



  • junbug20

    So that’s what happened! A lot of kids use because some family distinction for relief, pot provides that, I know, I was a heavy User every day, since 13 it was fun, a lot of fun, IQ drop? Don’t know about that, so you think there was some kind of brain damage? Depression anxiety I believe more so than the ability to solve problems or learning.

  • Samuel71

    There is no reason to pluralize “methamphetamine.” This is a common mistake that irks me to no end.

  • John Lawrence

    How were cannabis dependent subjects diagnosed?

  • John Lawrence

    It’s not that I’m an insomniac, it’s that I’m writing from a different part of the planet.

  • Pippa

    re 9 – junbug20, Just think how bright you might have been without smoking all that weed! As for Professor Terrie Moffitt, of the Institute of Psychiatry at King’s College London, I believe what he actually said was that for MOST people cannabis does not have long term effects on their brains. This of course does not rule out effects in a small subgroup with genetic susceptibility to psychosis – who are likely to become psychotic earlier and with a more severe form of illness, or the other physical effects of smoking it such as lung cancer, etc. Still, it probably is little worse than tobacco, medically speaking.
    Interesting adverts that go along with this article – mostly for drug rehab!

  • Miss Nancy

    Since it is a proven FACT -Harvard study, 2012 – that FLUORIDATION of WATER, which occurs throughout the U.S., reduces IQ, then how did this study account for THAT? Back to the drawing board! We’ve now got chemtrails dumping aluminum, barium, and strontium on our heads – all sorts of horrible illnesses are on the increase. We’ve got aspartame in the sweets, excitotoxins (MSG) in the fast /processed food, etc., etc. You cannot do FAIR studies on people anymore because there are just too damn many external factors that can bias results.

  • Ron

    Would it have been news if the study had been about alcohol? Where are the stats regarding alcohol, the preferred drug for the under eighteen crowd?

  • jake

    Lol it’s terrifying how quickly the stoners come out the woodworks to defend drug use and dismiss this study.

  • notjonathon

    Not a very convincing study, particularly given the very small number of so-called “dependent” users, and suspiciously timed, considering the number of marijuana referendums on US ballots this fall.

  • TheRant

    The people who love to read these kinds of articles and try to make them say “POT IS GREAT! POT FOR EVERYONE!” Kind of push me away from supporting the cause. They make it seem like a frenzy as oppose to a rational push for legality based off evidence.

  • FrogTale

    Pot made me more sensitive and no less clever when I was a teenager.
    I enjoy cannabis much more than alcohol.
    Prohibition of alcohol was a stupid failure, just like criminalizing cannabis use nowadays.
    I wonder how long it will take for courageous statesmen worldwide to end prohibition & legalize cannabis ?
    Wishful thiking my boy: Courageous Politician is an Oxymoron.

  • Daviticus

    Yes! TheRant gets it!! :)

  • MrsRobinson

    First thing that strikes me in this “study” is that is restricted to just about one thousand Australian kids. Second is that it keeps saying “pot dependance” when its a well known fact that there are no addictive properties in pot. Third, its long been debunked that pot is a “gateway drug” its simply the most available and widely used/sampled plus it stores in your body fat to metabolize over 30+days so traces of it can be found even in nontokers. Forth, puberty has been linked to a drop in IQ… you really think “5% of one one thousand Aussie kids” is a suitable representation of the other 7 billion of us… okay.

    Horomones and sex drive do more lasting damage than weed. I’m ashamed to see this trash blogged about as it were legitimate science on this website. Pot, like beer, has been in use for so many thousands of years that our physiology has adapted to it. The US government only accepts ‘pot studies’ when one of their three strains is used. And not to sound all conspiracy but think about the impact on the paper, oil product, clothing, fuel, medical industries and more if pot were legal.

    I’m sick of the slander. Real problems need to be tackled and too many misinformed morons are attacking weed like its destroying civilization.

  • pat oconnor

    I just happened on the Discover website and I am generally impressed at the level of discourse among the commentors and the lack of personal attacks, even when there is complete disagreement. MrsRobinson attacks morons in general but doesn’t call out anyone in particular in the discussion. This is refreshingly civilized. Thanks to you all.

  • Edward

    I’ve read the full study, and am familiar with the science related to epidemiology.
    This is the first “gold standard” study of the effect of marijuana on people that used a prospective, longitudinal approach in a study group of over 1000 in the cohort. This study group has generated over 1000 research reports in the 35 years of the study, and vast amounts of knowledge have been generated from this carefully done work. This study is important, and its results are very impressive.
    The researchers were able to demonstrate that marijuana use is associated with drops in neuropsychological functioning across a wide variety of tests. Over two thirds of the domains of intellectual functioning were found to be affected. While the drop in IQ was greatest for those with the earliest onset, greater use, and longest use, it was detected in those who never used as adolescents as well. The researchers factored out the effects of schizophrenia, alcohol, tobacco, and educational level. Brain dysfunction was even measurable in subjects who had not used marijuana in 20 years. This research has been largely underappreciated by journalists for its strength and influence among scientifically oriented people. Soon, we will have the same divide as anthrogenic global warming: only the scientists with a vested interest in legalization or medical uses will be advocates. Marijuana’s target organ for damage has been demonstrated to be the brain, and the damage is persistent even after use. The effect size of this intellectual drop is very large, and quite comparable to the effect of some chemotherapy/radiation combinations used to treat cancer. No other recreational drug of abuse comes close, except inhalants, of course. There is good reason to avoid it for everyone, and to absolutely remove it from the presence of our young people, for all our sakes.

  • SureThereMate?

    Anyone with knowledge/intelligence who can comment Edwards answer above?

    I have some comments. Are you saying weed is worse than alcohol? ecstacy? speed? coke? Really???

    What about studies showing damage is reversible, and effects gone after 28 days. Even in long-term heavy users. Comments here?

    What about the correlation illegal -> bad environment. Weed remains illegal, which definitely has major consequence of who become users. Also what users experience, both to the people they buy from, and anxiety from it being illegal.

    I would love to hear how you know so much about the study. Please give me information, because if what you say is true, I really want to acknowledge that. But I cant based on what I know now.


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