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?

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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? 

Peter Reynolds
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!

Muzzy Lu
Muzzy Lu

38 people who smoked marijuana heavily? What a large study! I guess it is much better to drink until you pass out like other teens! Great $2.99 e-book on medical marijuana: MARIJUANA - Guide to Buying, Growing, Harvesting, and Making Medical Marijuana Oil and Delicious Candies to Treat Pain and Ailments by Mary Bendis, Second Edition. This book has great recipes for easy marijuana oil, delicious Cannabis Chocolates, and tasty Dragon Teeth Mints. 

Notauser
Notauser

Using marijuana apparently alters reasoning and logical thinking because marijuana users are unable to accept the advice of our best doctors and scientists. The marijuana user whose mind is chemically skewed will attack anyone who suggests their drug of choice is self destructive this reaction is similar to jerking the pacifier from a baby's mouth.

Notauser
Notauser

Its a mind altering drug just like alcohol tobacco and many others. If you like to get high that's fine with me but its criminal to tell our young people that its safe and healthy.

Notauser
Notauser

Medical marijuana users take modern medicine back a hundred years where we were making remedes from tree bark and roots of plants.

cheap clothes
cheap clothes

Not only Marijuana, I think all kinds of drug are so harmful  for teenager. And it must be create bad affect of their brain and IQ. So  we should try to stop this.

Duncan20903
Duncan20903

Now, now, you can't really fault a prohibitionist for disseminating hysterical rhetoric. Hysterical rhetoric is is a fundamental part of prohibition.  What kind of girly man war on (some) drugs would we have without hysterical rhetoric?

JWR97
JWR97

Smoking more weed does not cause people to be dumber. It's the dumber people who smoke the weed in heavy amounts.  Please go back to 5th grad and learn the difference between correlation and causation. 

(Ex There is more crime in the summer time. There is also more ice cream consumption in the summer time. This does not mean that ice cream is causing people to commit crimes.)

 

People with low IQ tend to abuse drugs in greater amounts because they have fewer opportunities in life (higher education, good career outlook, etc). They also tend to have parents with low IQ who are not supervising them or enrolling them in extracurricular activities, where they would be busy and thus not sitting at home with the option of smoking weed all day.

This junk disguised as science is not what one expects to see in a national respected journalistic source. Pathetic.

MistaC87
MistaC87

I can sense B.S. all over this and I don't buy it for one minute. 

Jon Hyst
Jon Hyst

"Addicted to cannabis".

this study just became unbelievable.

Try harder big Pharm...

pmanpman
pmanpman

Speaking for the pot-smoking Americans who go to work everyday, take care of their family/loved ones, and abide by the law (except for the ridiculous one saying you cant smoke a plant that grows in the ground; unless of course its tobacco and it gives you cancer) I find this study ridiculous, you are taking people, who already barley meet  the 50th percentile or the middle of spectrum.  If you want to accurately determine the effect Marijuana has on Intelligence, Reaction time, and ability to comprehend/ retain information for set periods of time; Take the people at the highest point of the spectrum, the top 10%. Give these people an exam/questions while sober, have them remember questions/selected words and then have them recall 10-15 minutes later.  After the results are recorded take the same people and have they ingest the Marijuana; after the people have taken the herbal medicine and have had 15 mins. to let it set in, administer an equally challenging but different test. Compare all of the results once 2 of the tests have been completed; Once without ingesting marijuana and once under the influence.  Take the data and compare the consistencies and the dependencies in the ability to answer the questions/how well the info learned was retained, and if any of the information in the reading portion, of the test, provided was understood and applied to later questions.  Find an average based on the amount of people tested.  After this is completed you could do the same exact process with the people falling under the bottom 10%, take the results from the bottom,  at the end of this testing; compare all of the final results  for the top 10% against the bottom 10%.  I bet there is very minimal effects of this so called "drug" actually decreasing intelligence.  their test is flawed if you test fro the  middle you will never get an accurate score, there is way too much fluctuation in the middle.  the fact being that even if you can prove that marijuana can effect the way we learn as people, you cant deny that marijuana is the safest naturally occurring substance  in our ecosystem without any requirements of  added chemicals or modification.  If America could actually see the positives form letting the states "go to pot"; maybe we wouldn't be worrying about our high unemployment rate/national medical  plans, because we would have enough momentum/industry developing the 50,00 by-products of  Hemp (the major ones being,: Fuel, fabric, paper, rope, food, medicine, .....) We would be able to become a stronger economic power house, and we would be able to do all of this on are own home soil!  God i hate people who can stop for two mins to see it from the other side. my message for those people: smoke a joint and shut the fuck up!

cYaSK8
cYaSK8

they forgot to mention the water fluoridation poisoning and the alcohol consumed as well. these sensational reports are to help big pharma losing it's market share to  medicinal qualities of cannabis.

Nancye Kirtley
Nancye Kirtley

The same old propaganda trotted out once again. The "War on Drugs" has ruined more lives that Marijuana ever could. Ask the People in Mexico about now. But hey, it makes money for the Prison for profit and Law enforcement and Lawyers so hey, that's what makes it worth it, right? All the money that has been spent, not a drop..in anything, except the "Average Guy and Gal in America's wallets." Sure we need this alright..like shooting ourselves in the foot.

STOP THE PROPAGANDA..LEGALIZE and EDUCATE. It's the fiscally RESPONSIBLE THING to do.

Kenmrph
Kenmrph

The technical term for the drop in IQ resulting from marijuana use is "getting stoned."

Go Og
Go Og

Yep that explains why 99 percent of the Pot smoking population voted for Obama.  Because I am sure they had lost more than just 8 pts in their IQ.  

Hinks
Hinks

Good ad for your friend.