Teens, researchers, reporters, and how cannabis affects their IQ

One of the big discussions on the internet (and here in comments) yesterday was this study from New Zealand that was reported in the media as proving that use of marijuana by teens resulted in lowered IQ.

Check out the “reporting” by NBC’s Tia Ghose:

Teens who smoke marijuana see their IQs drop as adults, and deficits persist even after quitting, according to a new study.

“The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” study researcher Madeline Meier of Duke University said in a statement.

The study followed 1,037 New Zealand children for 25 years. Subjects took IQ tests at age 13, before any of them had smoked marijuana, and again at age 38. Throughout the study, participants also answered several surveys about their drug use.

My first reaction was “New Zealand? Why is it always New Zealand when they find something bad about cannabis?”

And it does seem to be true. New Zealand was where they did the tiny study that seemed to find a lung cancer connection despite the huge study in the U.S. finding no negative links at all. And there have been others.

Maybe it’s a difference in how the rigor of research is applied in Kiwiland.

Or could it have something to do with New Zealand in general? Maybe things are just really different there. After all, it’s a part of the world where some mammals have pouches, where a cute bear-like creature spends its entire life stoned on eucalyptus leaves, where birds that look like crows sound like babies crying and another bird laughs at you, where a vine can eat a tree and become a new tree, and where hobbits and wizards alike extoll the virtues of the local smoking materials. Who knows what that does to pot or pot-smokers?

Of course, the truth is that the study isn’t quite as big a deal as some of the reporters would lead you to believe. For instance, the lead sentence by Ghose: “Teens who smoke marijuana see their IQs drop as adults” is false. The implication is that 18 or 19-yeaar-olds who tried marijuana could see a drop in IQ, whereas the study only found differences in younger teens (ie, those under 18), who were regular or heavy smokers.

Maia Szalavitz explains why the breathless reporting is out of place here. Does Weekly Marijuana Use By Teens Really Cause a Drop in IQ?

Not all experts agree, however. “Scientifically, these are extremely preliminary findings,” cautions Carl Hart, associate professor of psychology at Columbia University, who has studied the cognitive effects of marijuana in humans in the lab and was not associated with the research.

Hart notes that because only 38 people in the study— around 8% of those who ever tried marijuana— used it heavily enough to get diagnosed with dependence during several follow-up periods, he is skeptical about how generalizable the results are. He says that in his studies of people who smoke at least three times a week, “When you compare these people’s scores to a normative database on a wide range of domains including executive function, memory, and inhibitory control, they score dead smack in the middle, in the 50th percentile.”

Of course, Tia Ghose never mentioned that only a small group in the study actually fit the criteria. (Just as they seldom mention that only a handful met the cancer criteria in the infamous New Zealand cannabis/cancer study).


Reacting to the study, Wim van den Brink, Professor of Psychiatry and Addiction at the University of Amsterdam said it was interesting research but its findings should not be overestimated. Speaking to Dutch daily de Volkskrant he pointed to the results from a sub-group in the study who stopped smoking the week before being tested. The effect on their IQ’s was much less pronounced. “The researchers are right to warn of the consequences of cannabis use at a young age,” he said, “but their results are probably being exaggerated.”

So, at this point, this is really nothing more than interesting research that should be viewed with caution in terms of any actual results.

However, even if the research is fully on target. If it is, in fact, true that heavy marijuana use by those under 18 can lead to lowered IQ, then it is an argument for legalization and regulation with age limits, not the status quo. I don’t know of any in the reform community who are pushing for heavy use of marijuana by children.

The more we have ramped up marijuana emforcement, the more we have ceded our ability to control age use of marijuana. School zones are a joke, because they have nothing to do with limiting use by children, but rather are a means of piling on charges for drug deals that didn’t involve them at all.

When I was in college in the 1970s, marijuana was illegal, but enforcement was much more lax. People regularly smoked pot in the dorms, and campus police walking by would merely sometimes ask that you close the door to limit the amount in the hallways. However, it was made very clear that if anyone sold pot to the High School students in town, there would be hell to pay. It was a clear line and one that was followed scrupulously. Oh, sure, I bet some High School students found a way to get some pot ocassionally (you’ll never be able to eliminate that entirely), but you can set up systems that say “above this age is deemed OK, and below this age is not” and make a real difference.

The best way to do that, of course, is through regulated legalization.

The one thing we know for sure from this study is that there is nothing in it that justifies arresting responsible adults.

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46 Responses to Teens, researchers, reporters, and how cannabis affects their IQ

  1. Mike Parent says:

    There were no controls in that study. The testers have no idea what else the subjects ingested over the years. To say it’s marijuana which caused the changes reported, is cherry picking results. Who funded that report?

    • claygooding says:

      “”shall ensure that no Federal funds appropriated to the Office of National Drug Control Policy shall be expended for any study or contract relating to the legalization (for a medical use or any other use) of a substance listed in schedule I of section 202 of the Controlled Substances Act (21 U.S.C. 812) and take such actions as necessary to oppose any attempt to legalize the use of a substance (in any form)””

      It sets no limit on the amount of money they can spend on false science,,procuring research requiring a targeted results is nothing new to our DEA/NIDA/ONDCP

  2. daksya says:

    From a Nature commentary on this study (h/t Transform):

    Valerie Curran: “Although the overall sample size is excellent, the data on adolescent onset of heavy use is based on just over 50 people.”

    She also noted that the findings represent a very small decline in IQ as a result of very heavy use over a number of years, “which doesn’t relate to recreational use”.


    Robin Murray: “Let’s keep some perspective. This is not huge,” Murray says. “You’re not becoming completely demented. Of course, it’s better to still have your extra eight IQ points, but it’s not something that’s enough to draw medical attention.”

  3. Francis says:

    From the perspective of your average prohibitionist parasite, an eight point IQ drop is absolutely huge. I’m not even sure it would be possible for most of them. (Can IQ be a negative number?)

  4. claygooding says:

    My thoughts on 13 year old’s smoking a joint,,,give them a cookie,,healthier way too consume,,,makes you wonder how mankind existed for thousands of years with cannabis available to anyone that desired it but the prohibition of it has increased the idiot ratio at the prohibitionist camps.

    • Mike Parent says:

      One has to wonder about the environment the 13 year old habitual smokers were brought up in. And did they consume other substances over all those years. The researchers make no mention about any other variables, yet they put marijuana use down as the factor which effected brain IQ. That’s right out of the Prohibitionist’s handbook.

      • darkcycle says:

        Mike, what is one thing almost all young drug abusers have in common besides drug use? Trauma. In nearly every case you will find psychic trauma. Abuse, neglect, abandonment, it will be there, somewhere in the child’s history. People who have experienced trauma are also more likely to experience depression. Depression, incidentally, can affect measured IQ in much the way it’s described here.
        And apparently, the folks who quit smoking ONE WEEK BEFORE THE TEST did not show the same loss.

      • TieHash says:

        From my long ago youth in high-school. The kids who were smoking freshman year were also inhaling solvents ( which cannot be good for the brain…and yet not prohibited )

    • kaptinemo says:

      Clay, I believe the damage runs a lot deeper than that.

      Prior to 1937, hemp products were practically a staple in society. It was everywhere. People were generally eating non-processed foods, and hemp oils and seeds were common in them.

      At the beginning of the last century, the incidence of cancer was one in 20. After cannabis prohibition, the incidence began to rise. In the 1960’s it was one in ten. Now? I’d say that you can practically count on getting cancer in your lifetime.

      Granted, there are a myriad of environmental factors involved, but one which has not been examined, for obvious legal reasons, has been the lack of cannabis-based foods in the American diet for 3 generations…and the incidence of cancer in America is becoming a coin-toss.

      Given what we already know about the de facto (if not de jure) conspiracy to ban hemp in favor of petrochemicals, I wouldn’t be surprised at all if this had been deliberate.

      • claygooding says:

        I have ranted before on the cancer blocking attributes of cannabis with four statistical studies done around the world that reports as much as a 60% lower cancer rates for certain types of cancer plus all the anti-tumor clinical studies show that cannabis has a direct effect on cancer cells,,which makes our government prohibiting cannabis and thus promoting cancers to people all over the world,,while big pharmacy has no cures or cancer blocking medicines,,,only treatments at $50k per treatment,,,it’s a sick world and holding the thermometer up it’s ass is a dirty job,,but someone has to do it.

      • War Vet says:

        Who would have thought antioxidants are good at fighting and reducing cancers? Hemp is saturated with antioxidants. I wonder if this study asked about Prozac or Ritalin being used by smokers and non-smokers when making this study . . . what about tobacco use -surely nicotine isn’t good for human brains . . . how many of these kids grew up in second hand smoke homes?

        • War Vet says:

          What about kids who use DEET bug sprays when out in the woods . . . how many of these kids drink water coming from pipes or lands that have been drilled for NG? How many of these kids drink water from plastic bottles -something said to be bad for human consumption.

    • Byddaf yn egluro: says:

      They’ve removed it —I guess it was starting to make too much sense. To those who took the trouble and contributed: Thank you!

  5. Bruce says:

    Another Dog down. Arooooooo! 75 yr old Woman wets herself. You Ba$tards. You get IQ score negative 2. Demons.

    • Windy says:

      I shared that on facebook with this comment:
      This is absolutely sickening, this is your “war on drugs” and the police state it has created, this is just a taste of the future if we do NOT end this travesty of a policy.

  6. Jonny Goldstein says:

    Thanks for the post, Pete!

  7. Liam says:

    Though I could not find it to cite, a recent article in Science Magazine noted that the vast majority of research findings are false. They attributed much of the reason to researcher bias, i.e., stopping the study when their preconceived expectations are confirmed.

    An older (2005), more dense, paper by John P. A. Ioannidis, Why Most Published Research Findings Are False, at:


    goes into greater detail. Something to keep in mind when reading purported “facts” and when taking your prescription meds.

  8. claygooding says:

    O/T but interesting,to us marijuana growers/enthusiast,,,vaporize your water/trim leaves,,since vaporization just melts the thc as hot air flows over the leaf,all you get is the good stuff,,and surprising is that it actually tastes pretty good.

    Beats having to smoke it cause your outta bud.

  9. ezrydn says:

    Yep, dat Cannabis is bad stuff. I smoked constantly while in college, leaving with TWO Ph.D.s. Then, turned around and obtained a Juris Doctorate. All the while, going through bag after bag of that IQ-destroying weed. I also found the time to acquire Fixed- & Rotary-wing Instructor ratings, which included Instrument Instructor. As a side hobby, other than my relaxing smoke, included qualifying for my Amateur Extra HAM ticket.

    Yes, it saps your IQ and will to achieve, as I’ve now proven. WHEN WILL THIS BULLSHIT END???!!!

  10. allan says:

    again… if we are to discuss the harm of substances it must be comparative. Let’s see similar study results from 13 and 14 year olds that started as heavy consumers of alcohol, cigarettes, meth (ritalin), etc. I’d also wonder about any family that (however) allows a 13 year old to consume any substance heavily.

    I also want to know how many of the subjects in the NZ study grew man boobs and had sexual dysfunctions… they can build the propaganda and now with the interwwweb we can expose and destroy the propaganda. Thank you Al Gore!

    and OT… from a couple of very fine cannabists/dpr types (for a cuppla fur’ners – Nimbin and Vancouver, BC) a new app for “reading” bedtime stories to kids, Little Car Stories. Child literacy, the cure for APE (Adult Prohibitionist Excrementalism). And as I would expect from these two, the stories are more than little car stories…

  11. Pingback: Teen Pot Smoking Linked to IQ Decline In Later Years - Page 4 - Grasscity.com Forums

  12. Duncan20903 says:


    It doesn’t seem to even slow down the production of prohibitionist propaganda when you point out that their own Monitoring the Future survey shows that youth use of drinking alcohol and smoking tobacco are at 30 year lows while youth use of cannabis is at 30 year highs.
    ———- ———- ———- ———- ———-

    Washington spent more than $200 million on marijuana enforcement in decade

    • claygooding says:

      I keep searching for the ad banner that has the pill for removing all common sense or the ability to ignore all information that shows conflicting evidence to the users agenda,,and not that just the prohibs are guilty of it but they seem to have a higher percentage of people that ignore science and common sense,IMO.

      • Duncan20903 says:


        That’s an inherent problem when your entire argument is based on a platform consisting of nothing but bald faced lies, half truths, and hysterical rhetoric. How the heck can they make an argument without ignoring the facts?

  13. Ned says:

    This kind of crap is designed to distract and obfuscate the real issue of certainty about the need to end prohibition EVEN THOUGH it unintentionally points toward legalization as a method of age limiting access.

    It’s the, “see, it’s bad for you so therefore it must remain illegal” idiocy.

  14. Patchword says:

    While I agree that problems with cannabis should not be addressed by legal means you might like to note you’re using our neighbours across the Tasman as source of evidence that we’re a rum lot. Australians and New Zealanders speak English and have quite a lot in common but do not share flora and fauna. Still we all share the Mother England heritage FYI.

    • Duncan20903 says:


      Both the US and Australia were founded by criminals. Recently I was wondering if there might be something about that that makes our respective governments so intransigently daffy.

    • Pete says:

      Sorry about that. I’m clearly not aware of all the cross-over or lack thereof. I got the hobbits and wizards right, though. And my sense was not to indicate that you’re a “rum lot” but rather that there are significant differences in lots of things on the other side of the world. You even have different stars in the sky.

    • Peter says:

      The origin of the Australian term for the English, P.O.M.E., (pronounced pomm-ee) stood for Prisoner of Mother England, after large numbers of English convicts were transported to found the first Australian colony, beginning in the 1780s. This became necessary after the War of Independence/Revoluntionary War prevented further disposal of convicts in America. Some 50,000 English, Scottish and Irish convicts were sent to North america between 1620 and 1776 and many, no doubt, were put to work growing hemp and cannabis.

  15. Peter says:

    Obama popped up on reddit with an offer to answer questions…i can’t believe for a moment that no redditors asked him the obvious question about his change of attitude to drug reform before and after becoming president. About the closest we got was a request for the recipe of his home brewed beer. Methinks there was some filtering going on… if he really wants to get the hipster vote he’s going to have to do a lot better than this:

    • Peter says:

      my mistake, there is some stuff further down the thread, but needless to say the drug warrior in chief failed to answer it:

      “Let’s skip the marijuana legalization question that’ll show up at least 50 times on this page and get to a related issue: After promising that you wouldn’t interfere with individual state decisions on medical use of cannabis, how can you justify utilizing federal funds and agencies to shut down dispensaries and arrest people who are legitimately sick?
      [–]SunshineToss 195 points 1 hour ago
      20 bucks says he doesn’t address this
      load more comments (19 replies)
      [–]shadowofthe 16 points 1 hour ago
      As a secondary point, what role do you think states’ rights (surrounding drug legalization) will play in the rest of your presidency?
      [–]barrister_bear 5 points 1 hour ago
      Amen to this. Many people voted for and supported you based upon statements you made regarding the drug war. What caused you to change your mind?
      Much respect and gratitude for getting on here and answering our questions.
      load more comments (16 replies)
      [–]eddieohare 246 points 1 hour ago
      What are you going to do in your second term to address the failed war on drugs?”

    • kaptinemo says:

      I’ve been over at RawStory most of the day, which is usually a good source of articles on cannabis, and the DLC shills are really out in force, making their “Vote for Obama or the big, bad, three-horned, six-eyed, sabertoothed Romneymontser will eat your first-borns!”.

      They adamantly refuse to admit the current crackdown is a betrayal (‘Battered Wife Syndrome’, anyone?) and are trying to shame and marginalize their critics as being ‘myopic’, ‘immature’, ‘single-issue voters’.

      Needless to say, the shills are catching holy Hell in the comments.

      • The question in my mind would be, why was Obama pandering in the last election to ‘myopic’, ‘immature’, ‘single-issue voters’?

      • Duncan20903 says:


        What I can’t believe is how many people seem to believe that Mr. Obama is simply playing the role of foaming at the mouth drug warrior to get re-elected, and that on November 7 he’ll start to dismantle the war on (some) drugs. I guess my drug rehab counselor that loved to say, “if you don’t stand for something, you’ll fall for anything” was right.

  16. B.Snow says:

    I think this little gem is possibly the “cleanest” example of a conflict of interest crossed with self-serving douche-baggery in single sentence I’ve seen in years

    “I think this is the cleanest study I’ve ever read” exploring the long term effects of marijuana use, Dr. Nora Volkow, director of the National Institute on Drug Abuse, told the Associated Press.

    I continue to be amazed by the levels the NIDA folks will stoop to = In order to insure their job security – pushing to keeping it in the “indefinitely” category.

    With all their self-fulfilling works of pseudo-prophecy & biased “anti-truth” only fund/do/allow research that starts with a thesis statement/objective of marijuana being harmful to people’s health.

    And how loudly they’re blowing the “think of the children” dog-whistle here… I think George Carlin had the best take on this crap – “The kid who swallows too many marbles doesn’t grow up to have kids of his own.” Only, it doesn’t have to be limited to “kids” & “marbles” either. It was equal opportunity wisdom!

    “Help the children, save the children, protect the children.” You know what I say? Fuck the children!

    What I’m talking about is this constant, mindless yammering in the media, this neurotic fixation that suggests that somehow everything–everything–has to revolve around the lives of children. It’s completely out of balance.

    If you remember this bit you know what I’m talking about & if not I suggest you google it up & watch it = Priceless Comedy… still hard to believe he’s been gone over 4 years now.

    • Duncan20903 says:


      I want to write a letter to Peter Lewis suggesting that he put a couple of his millions into research. I’ve read somewhere that he actually reads and responds to his mail. Anyone care to help me compose it?

    • Mike Parent says:

      Legalize and Regulate!
      If they really cared for the children they’d legalize and regulate marijuana. If they really wanted to keep any substance out of the hands of “The Children” they first must take control of distribution away from black market dealers. They haven’t accomplished that in 40+ years at a taxpayers cost in the hundreds of billions. It’s time to treat marijuana as we do alcohol. My 27 year old daughter still gets carded when she buys alcohol, yet your 13 year old can buy anything the black market dealer has for a price whether it be money or “something else”.
      FACT: Your kids have a better chance dying at the hands of someone enforcing marijuana laws than they do from ingesting it.(ZERO %).

      LEAP member, NYPD, ret.

  17. SCOOBY says:

    And what ??!……..the legal drug “alcohol” has no effect on the brain ??!…..I’ll tell you what makes my brain dull is the constant and never ending line of bullshit from our quote “leaders” unquote.

  18. Nibhana says:

    Marijuana harms your short-term memory, but if you ge off it for a couple of weeks, that’s fixed. Short-term memory is working memory, and it is a huge part of fluid intelligence, so it would make sense that using it all the time would hurt you. The good news is that working memory can be increased easily, by doin dual n back or digit span exercises, and that those increases translate into increases in IQ. The other side of marijuana’s effect on intelligence is that weed kills your motivation. This is the worst part, because if your synapses aren’t exercised, they’ll be pruned, and you’ll lose knowledge and intellect that you possessed previously. Used moderately, these two effects don’t do much, and the neuroprotective effects can become useful. In adults, moderate marijuana usage has been shown to cause an iq increase of generally 5 points. While that’s not much, and while I wouldn’t suggest mj over piracetam, it still shows that it can be useful as long as it isn’t being overdone. My hypothesis is that there is an optimal level of consumption, in which one has the motivation one normally has, and the neuroprotective effects weed has. Since pruning is nearly inevitable, I believe that those effects may lower the amount that normally occurs with a typically stimulated brain. If pruning occurs at baseline iq levels, but doesn’t change it with normal stimulation, the. It would make sense that inhibition of pruning would actually cause a net increase in IQ. In short, weed can be useful if dosed correctly, but one needs discipline to do so, discipline most pot smoking teens just don’t have.

  19. Normally I do not learn post on blogs, however I would like to say that this write-up very forced me to check out and do it! Your writing style has been surprised me. Thanks, very nice post.

  20. allan says:

    oh look, mom has packed a bag lunch – with a SPAM sandwich!

    And Pete… I don’t know if “Your writing style has been surprised me” is a compliment or not.

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