Does Marijuana Use by Teens Really Cause a Drop in IQ?

A new study suggests marijuana use could have an impact on American teens' IQ, but how great is the effect?

  • Share
  • Read Later
Getty Images

Heavy marijuana use is associated with cognitive decline in about 5% of teens, according to a new study, which suggests that the heaviest users could lose 8 IQ points.

In the report, published in the Proceedings of the National Academy of Sciences, research conducted in New Zealand showed that teens who started smoking marijuana before age 18 and were diagnosed as being addicted to cannabis by age 38 experienced an IQ drop in early adulthood. But users who began smoking after age 18 — even if they used heavily — did not show a significant decline.

“The effect of cannabis on IQ is really confined to adolescent users,” says lead author Madeline Meier, a postdoctoral researcher at Duke University. “Our hypothesis is that we see this IQ decline in adolescence because the adolescent brain is still developing and if you introduce cannabis, it might interrupt these critical developmental processes.”

The authors followed 1,037 children born in the town of Dunedin, New Zealand, in 1972 and 1973 — virtually every child. They defined adolescent use as at least weekly use before turning 18. In looking at the relationship between marijuana use and IQ, they controlled for factors like years of education, schizophrenia and use of alcohol or other drugs that might also have an effect on IQ. While education weakened the relationship, it still did not eliminate it.

Researchers also had family members and friends of the participants confidentially rate them on attention and memory skills and those who had lost IQ points showed problems in these areas. Meier notes that an eight-point decline in IQ for someone with average intelligence (an IQ score of 100, the 50th percentile) would move that person down to the 29th percentile. “It’s fairly substantial but it does depend on where you start out,” she says.

“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.

(MORE: Can Addictive Behaviors Be Predicted in Preschool?)

“The overall implication is that when you’re talking about marijuana, you have to take into account age of onset of use and dealing with developing, growing brains,” says Meier.

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. [Disclosure: Hart and I are working on a book project together.]

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.”

He explains: “They are normal when not intoxicated. We test them when they are not intoxicated and when they are intoxicated. When they are intoxicated, there is some slowing of certain cognitive acts, but their accuracy doesn’t not change.” The New Zealand study, for example, did not identify whether the participants were employed or whether they were able to function in their families, which would be an important indicator of whether the drop in IQ has any real-world impact.

There are also other factors — such as child abuse or other trauma — that might lead people to seek escape in heavy marijuana use and could also affect brain function. Meier and her colleagues did not examine these factors but say it’s possible that such elements could explain the results better than marijuana itself.

If the link is real, the effects on cognition could be dramatic. But intelligence and cognition is affected by a plethora of other factors, including genetic, social and environmental influences that may supersede any influence from drug use. Despite the fact that the average marijuana user starts at age 17 in the U.S and nearly 7% of high school seniors currently smoke pot every day, IQ scores have risen tremendously over time in all developed countries in recent years. Most of those same countries also experienced a massive increase in marijuana use between the 1950s and today.

MORE: 10 Reasons to Revisit Marijuana Policy Now

Maia Szalavitz is a health writer for TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland‘s Facebook page and on Twitter at @TIMEHealthland.

98 comments
Kadak
Kadak

Cannabis is  foriignorant idiots

ShaunJones
ShaunJones

its a pretty obvious fact to anyone that smokes alot or heavily has trouble remembering things, so ofcourse it effects cognitive function, same way trying to learn a new skill while stoned is almost impossible. but then you stop smoking for the day and find your learning ability is back to normal. ive smoked since i was 17 and im now 27 i have no complaints and i do think it should be legalized, but i do think that its the majority of pot smokers are the ones making our case harder, chav kids hanging on street corners smoking weed and drinking then causing trouble because the first thing they blame is the weed because its a banned substance, when its obviously the alcohol. teach people how to handle it better, rather than telling them 'dont smoke'

annapiantino
annapiantino

@andrefgm sejam por causa de traumas que levaram ao uso, e não por causa da maconha propriamente dita

annapiantino
annapiantino

@andrefgm mas ou, n gostei de uma coisa desse artigo... quando eles falam que talvez os "brain damages" dos viciados em maconha

Notauser
Notauser

All you potheads who think its OK for young people to use marijuana are your own worst enemy. If you think for one minute that the American public is going to ever vote to legalize minors using marijuana then you must be smoking some good bud because that will never happen. 

If and when marijuana is legal it will be legal for adults only not minor children.  Keeping the argument going that pot is OK for everyone including children will not advance your  cause.

jway
jway

It's the most bored people who smoke the heaviest amounts. The researchers should look at what makes people bored (lack of money, lack of opportunities) and try to correlate that with IQ.

Anyway, the most interesting thing about the research was that people were able to get marijuana at all. Marijuana is ILLEGAL in New Zealand, and the fact that so many people were able to get marijuana is a damning indictment on the FAILURE of the prohibition!! If it DOESN'T WORK then it doesn't matter if EVERYBODY ended up with a lowered IQ - keeping marijuana illegal didn't make ANYBODY safe!! (it did make the criminals nice and rich though)

We'd be far better off legally regulating marijuana like beer and wine and letting drinkers switch to this FAR safer recreational drug. Legalization rid our streets of bootleggers and legalization will rid our streets of drug dealers.

~~~~~~~~~~~~~~~~~~~~~

Before Iraq

Before Vietnam

Before WW2,

They started the Drug War.

It STILL doesn't work!

~~~~~~~~~~~~~~~~~~~~~

AlexanderGalt
AlexanderGalt

Decriminalization of cannabis only makes sense if cannabis is as benign as its apologists claim.

 

But as well as being as addictive as tobacco according to the Royal Association of Psychiatrists and responsible for many deaths marijuana causes psychological damage which puts it in a different league from the pedestrian killers, tobacco and alcohol.

 

How is dealt with at greater length here:

http://john-moloney.blogspot.c...

Justin Davenport
Justin Davenport

This was a paid study by a political organization funded by anti marijuana companies. It should be completely discredited. It does not cause iq loss at all. It is one of the most powerful medicines in the world that can cure cancer cells and tumor's in the brain. 

Notauser
Notauser

The potheads who come here and refute this study online are despicable. If there is even the faintest possibility that the study could be true young people under 18 should be discouraged from using marijuana. Lets not experiment with the mentality of a whole generation of people. My god we don't need more dumb people. 

Chris Carter
Chris Carter

Has anyone who has looked at the free supplementary data in this report noticed that the heaviest cannabis users consistently showed a marked increase in their arithmetic intelligence as well as in other categories like picture completion tests. Not only is their an increased intelligence in heavy cannabis users relative to non-users in these categories, but one could say these categories test natural intelligence better than the information tests (which ask questions about geography and such) and vocabulary tests (stratightforward vocab knowledge).. Does this suggest that for enhancing some types of intelligence heavy cannabis use is advisable?

hanspy
hanspy

A new study suggests marijuana use could have an impact on America's IQ, but how great is the effect?"QuoteOooh.Is this all?Its about American IQ levels.Thats not strange.They have already proven to have lost loads IQ points the past decade.

Andrew West
Andrew West

LMFAO, Yeah right, I have smoked marijuana daily since I was 14. I don't get high, I stay high. I have been in honors my whole life, and received two awards for scoring one of the highest scores on a statewide test. For this one study, there are dozens more that prove people with higher I.Q are more likely to try drugs. The reason being their willingness to learn what they don't understand, and not believing something just because they were told it was true. In countries that allow testing of this medicine, it is currently being tested not only as a cure for cancer, but as a treatment and possible cure for Alzheimer and Dementia. Studies show again and again that marijuana use alone does not kill brain cells, on the contrary, near every study shows it helps our brain build new brain cells and new pathways. For those of us with intelligence, try again. Reefer madness has failed, some of us have woken up. This kind of nonsense will not work anymore.

yourmom5
yourmom5

Great analysis, it's likely that poor diet or addiction to comic books or sports could reach the same or worse results. It disgusts me to see other news outlets publishing yellow journalism in regards to this one small study. Very shady goings on.

Joseph Rivera
Joseph Rivera

Kevin you have to stop making sense and providing facts because it doesn't correlate with the image of a cannabis smoker. 

I Guess its hard to acknowledged cold hard facts 

JohnOBX
JohnOBX

I hate being around potheads the same way I hate being around drunks.  Both act equally stupid and are equally annoying.  The only thing worse is a drunk and stoned, flag-draped tea party sycophant.   

Whatnow05
Whatnow05

Just listen for a stoner laugh...  Though the drop in IQ could just be short term memory loss. 

If it didn't effect you why else would someone do it?  No free rides in nature getting high or getting drunk or too much of anything will impact you. Granted some things worse than other, but lets not pretend that there are no "side effects"...Once again what would be the point then? 

Kaoss
Kaoss

Your reasoning to legalize stems from your experience, and therefore is invalid for everyone else; and it applies only to you. This is a scientific study, and therefore has gone under rigourous examination by various professionals. IQ scores do not change significantly from birth to death, and you will cause lifelong learing issues and thus impact the society at large.

ShaunJones
ShaunJones

@Notauser and you naysayers who judge pot smokers, not potheads, thats just another insult from the non smoking louts that have nothing better to do or say.

i guess you honestly dont realise how many people today smoke marijuana, i honestly think your living in a dreamworld thinking that the world isnt already closer to legalizing it. i also think you have missed alot of news seeing as california for instance is using it medically, oh and that includes kids.

seriously your so deluded from years of propaganda that you obviously think its actually bad for you. i think you should take the time to actually read up on what your talking about as you obviously have no clue. the benefits far far far outweigh the cons, of which they are barely even exsistant, it helps remove cancer cells....... need i say any more? if thats not enough to make you actually read up on something instead of just assuming like any other typical everyday naysayer, then i dont know what is.

feel free to continue living in your 1960s bubble of 'if i dont look into it, then i can just assume propaganda is right'

Steve7
Steve7

@jway yes! and why did they look at adolescents in the 70s? It's almost common knowledge that marijuana has changed chemically in the past 40 years

esotericknowledge
esotericknowledge

Or maybe they are truth tellers and you are trying to deceive?

Michael Novak
Michael Novak

 your so ignorant you won't even listen when kids (with blatant pride) tell you its easier to get pot then alcohol. i swear; the only thing left is for the actual plant matter to grow lips and inform you putrid pot prohibitionists. by the way something smells fishy with that study. im not proclaiming deceit. im just saying the human brain doesn't stop growing at age 18, but more in the early 20's. so what's the connection between lowering young developing kid's IQs and not older developing kids. although i suppose it could just mean more of the crucial developing starts earlier.

Kevin Hunt
Kevin Hunt

"The potheads who come here and refute this study online are despicable."Right, because your view of science dictates that one study proves a scientific theory beyond a shadow of a doubt and makes it scientific law.  Prohibitionists always seek to stop the debate because they are afraid that 75 years of  non-science with regards to marijuana studies will be exposed.  Go ahead and call your opponents "despicable", you won't stop the debate.

Kevin Hunt
Kevin Hunt

 "People

who have been drinking large amounts of alcohol for long periods of time run the

risk of developing serious and persistent changes in the brain. Damage may be

a result of the direct effects of alcohol on the brain or may result indirectly,

from a poor general health status or from severe liver disease."http://pubs.niaaa.nih.gov/publ...

Jennifer Herrald
Jennifer Herrald

Andrew, what do you know about scientific studies? Have you heard of randomised controlled trials? Rigorous and valid research is not based on observing one person. Comparing one person with himself can only tell you something about that person, and even this information may not be valid if the study does not meet certain conditions. There will always be exceptions, and research results typically indicate the chance of likelihood (in the form of a statistic), as opposed to unequivocally asserting what will happen in 100% of cases.

Notauser
Notauser

Andrew you sound like a very smart guy and I commend you on your achievements. You say there are many studies that show that marijuana builds brain cells and is a possible treatment and cure for Alzheimer, dementia and cancer.  Will you please post links to the studies you are talking about?

Notauser
Notauser

Apparently you are in the group who has experienced an IQ drop.

Notauser
Notauser

please have another bong hit.

Kevin Hunt
Kevin Hunt

 Outlaw alcohol = no more drunks.  It worked from 1919-1933, right?

nickothime
nickothime

@Kaoss This "study" is so scientifically flawed as to be a re-run of Refer Madness.

ShaunJones
ShaunJones

@Kaoss what are you talking about? seriously talk about back to front logic, you think people with experience in something would know less than those that have no experience whatsoever? see this is the actual problem with it, silly ideals by people such as yourself that actually think so back to front that entire groups of people end up hating something they dont even understand. pointless even debating it with you if thats your view on experience.

Notauser
Notauser

Or maybe you are high right now and confused.

AndrewWest
AndrewWest

@Jennifer Herrald See my replies above to Notauser

wooginowski
wooginowski

@Jennifer Herrald Why does the United States own a patent for this?

AndrewWest
AndrewWest

@Notauser I had to pull that patent link as it says in straight writing "Cannabinoids have been found to have antioxidant properties, unrelated to NMDA receptor antagonism. This new found property makes cannabinoids useful in the treatment and prophylaxis of wide variety of oxidation associated diseases, such as ischemic, age-related, inflammatory and autoimmune diseases. The cannabinoids are found to have particular application as neuroprotectants, for example in limiting neurological damage following ischemic insults, such as stroke and trauma, or in the treatment of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and HIV dementia. Nonpsychoactive cannabinoids, such as cannabidoil, are particularly advantageous to use because they avoid toxicity that is encountered with psychoactive cannabinoids at high doses useful in the method of the present invention. A particular disclosed class of cannabinoids useful as neuroprotective antioxidants is formula (I) wherein the R group is independently selected from the group consisting of H, CH.sub.3, and COCH.sub.3."

AndrewWest
AndrewWest

@Notauser Yes I can actually, and would be happy to do so. Here is a link for a page from the National Cancer Institute. If you read it, it will explain how marijuana kills bad cells, while protecting healthy cells. It explains they don't fully know how this works but it does. At the bottom they reference 30 something studies on the subject including, cannabis eating tumors, it's protective effect on brain cells, and a whole host of other benefits.

Michael Novak
Michael Novak

 its always fun watching prohibitionists jump the gun.

( ah, someone said something stupid. "pothead". oh wait, that's a mirror.)

Kevin Hunt
Kevin Hunt

 Apparently you have nothing to say but insults, which is indicative of an immature level of physical and/or social development.

yourmom5
yourmom5

Either way it's obviously still leaps and bounds above yours. In fact your extreme inability to comprehend the most basic understanding when looking at preliminary studies in that correlation does not equal causation says a lot about your mental capacity. Continue to enjoy your comic books, sports, and cheeseburger addiction.

yourmom5
yourmom5

And apparently you are a helmet wearing, anime porn addict, with an insatiable appetite for verbal masturbation. Probably cheeseburgers too. Go tell your mom its time for your nap.

Michael Novak
Michael Novak

 or maybe he's confused and not high. way to retort everything back to "that guy is stoned". i swear, who are the ones that are suppose to be addicted? us pot smokers or you ignorance slingers?

Kevin Hunt
Kevin Hunt

From IMDB: Tommy Chong is credited for acting in 35 films and shows,  writing 11 movies, 11 soundtracks, producing 3 shows, and directing 6 movies.

Cheech Marin has credits for 98 movies and shows,  writing 18 movies and shows, 16 soundtracks, producing 4 shows, and directing 2 movies.

What was all this nonsense about "reduced mental ability"?

yourmom5
yourmom5

Cheech and Chong happen to be multi-millionares that wrote and produced their films. There are two sides to every coin and steoreotypes are rarely worthwhile or justified. They are simply a way to a generalize and marginalize a group of people with opposing views. Out of the 1000 participants in this study they only were able to find 38 that fit the criteria of depedence. In fact the ones with the largest increase in IQ were those who had some experience with marijuana not the ones who never smoked.

Notauser
Notauser

The general public perception of a heavy pot user is someone with reduced mental ability a funny clown who says dumb things like Cheech and Chong. 

I am not surprised if there is some truth to to it.