Study: Adolescent marijuana use leaves lasting mental deficits

The persistent, dependent use of marijuana before age 18 has been shown to cause lasting harm to a person's intelligence, attention and memory, according to an international research team.

Among a long-range study cohort of more than 1,000 New Zealanders, individuals who started using cannabis in adolescence and used it for years afterward showed an average decline in IQ of 8 points when their age 13 and age 38 were compared. Quitting pot did not appear to reverse the loss either, said lead researcher Madeline Meier, a post-doctoral researcher at Duke University. The results appear online Aug. 27 in PNAS.

The key variable in this is the age of onset for marijuana use and the brain's development, Meier said. Study subjects who didn't take up pot until they were adults with fully-formed brains did not show similar mental declines. Before age 18, however, the brain is still being organized and remodeled to become more efficient, she said, and may be more vulnerable to damage from drugs.

"Marijuana is not harmless, particularly for ," said Meier, who produced this finding from the long term Dunedin Multidisciplinary Health and Development Study. The study has followed a group of 1,037 children born in 1972-73 in Dunedin, New Zealand from birth to age 38 and is led by Terrie Moffitt and Avshalom Caspi, psychologists who hold dual appointments at Duke and the Institute of Psychiatry at King's College London.

About 5 percent of the study group were considered marijuana-dependent, or were using more than once a week before age 18. A dependent user is one who keeps using despite significant health, social or family problems.

At age 38, all of the study participants were given a battery of psychological tests to assess memory, processing speed, reasoning and visual processing. The people who used pot persistently as teens scored significantly worse on most of the tests. Friends and relatives routinely interviewed as part of the study were more likely to report that the persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.

The decline in IQ among persistent cannabis users could not be explained by alcohol or other drug use or by having less education, Moffitt said.

While 8 IQ points may not sound like a lot on a scale where 100 is the mean, a loss from an IQ of 100 to 92 represents a drop from being in the 50th percentile to being in the 29th, Meier said. Higher IQ correlates with higher education and income, better health and a longer life, she said. "Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come," Meier said.

Laurence Steinberg, a Temple University psychologist who was not involved in the research, said this study is among the first to distinguish between cognitive problems the person might have had before taking up marijuana, and those that were apparently caused by the drug. This is consistent with what has been found in animal studies, Steinberg added, but it has been difficult to measure in humans.

Animal studies involving nicotine, alcohol and cocaine have shown that chronic exposures before the brain is fully developed can lead to more dependence and long-term changes in the brain. "This study points to adolescence as a time of heightened vulnerability," Steinberg said. "The findings are pretty clear that it is not simply chronic use that causes deficits, but chronic use with adolescent onset."

What isn't possible to know from this study is what a safer age for persistent use might be, or what dosage level causes the damage, Meier said. After many years of decline among US teens, daily use has been seen to increase slightly in the last few years, she added. Last year, for the first time, US teens were more likely to be smoking pot than tobacco.

"The simple message is that substance use is not healthy for kids," Avshalom Caspi said via email from London. "That's true for tobacco, alcohol, and apparently for cannabis."

Explore further

High childhood IQ linked to subsequent illicit drug use

More information: "Persistent Cannabis Users Show Neuropsychological Decline From Childhood to Midlife," Madeline H. Meier, Avshalom Caspi, et al. Proceedings of the National Academy of Sciences, Online Early Edition, Monday, Aug. 27, 2012.
Provided by Duke University
Citation: Study: Adolescent marijuana use leaves lasting mental deficits (2012, August 27) retrieved 18 January 2019 from
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User comments

Aug 27, 2012
more propaganda. you guys cant keep it outlawed forever as hard as you try. these "scientist" are nothing more than government dogs. cherry pick away boys....

Aug 27, 2012
All the more reason to legalise it. Prevent under-18s from using it. It has been demonstrated that under-18s have greater access to cannabis than alcohol and tobacco and it makes sense since drug dealers do not ask for identification. Drug dealers do not care how old you are. Additionally, giving cannabis the status of illegality only compels people (especially adolescents) to use it. Nobody likes being told what to do. And nobody should be told what to do with their bodies. So cannabis results in decreased cognitive function. So what? Lack of exercise results in decreased hippocampal neurogenesis. Junk isn't good for you. Too little sun exposure gives you rickets, too much gives you melanoma. How much legislation are we willing to commit to instead of giving people the freedom to make their own choices?

Aug 27, 2012
This IQ test sounds dodgy. Such large claims made from a small sample of people. You tested a room full of idiots who smoke marijuana, then claim everybody who smokes marijuana as a minor is dumber than anyone who hasn't. What were these IQ tests? Can i sit one of these tests?

Aug 27, 2012
Strange! ...My tested IQ went from 138 in 1961 US Army AFQT's at age 18 to 163 at age 38. Test on record at NASA. Tested by a cognitive perceptual psychologist at The Human Factors Research Lab. To be fair the docs said I was hard wired differently than most. I am a High School dropout with 6 ASAIO published articles in the Nat Lib of Medicine. Also hold an Airline transport pilot certificate and one of very few westerners type rated in Soviet Mig25 and Sukhoi24M D variant fighter aircraft. So next time pick some breathing subjects for your tests.

Aug 28, 2012
It may be that the situation is being looked at from the wrong direction. Marijuana is seen necessarily as a potent drug that is destroying good people. Maybe not.
Arton's remark demonstrates a great deal, saying the popularity of marijuana is because it's forbidden and that "only compels people (especially adolescents) to use it". "Nobody likes being told what to do", Arton adds.
In fact, that is all wrong. Only imbeciles are compelled to use something just because it is denied! People of even borderline normal intelligence recognize when being told something is genuinely for their own good. Marijuana enjoyers are no more advanced intellectually than the teens who videotaped themselves putting their hands in rat traps and springing them. Normal people don't engage in the kind of recreational marijuana use Arton and such are endorsing! Those who do are "ruined", but they didn't have far to fall to failurehood to begin with.

Aug 28, 2012
How does this IQ relate to marijuana? It doesn't. It's strictly correlational. How do we know that an 8 point IQ drop isn't normal for large segments of Kiwi society? Maybe they're painters or drinkers or smokers or conservatives. Maybe they're predisposed to headbanging or rugby or football, banging their heads like hammers. Maybe the junk food they eat rots their brain. No casual connection whatsoever.

Aug 28, 2012
Bad science isn't news...

Aug 28, 2012
Kochevnik, Since Progressives and Liberals encourage and even glamorize smoking POT (go to hemp fest in Seattle and ask the people what party they support), it shows that either progressives have lower IQ from the start or explains why they have low IQ.

Aug 28, 2012
I think most everybody above has covered the topic in every other way concerning Marijuana.

As this is a correlation only type of "confirmation" or truthfully: a useless "study" for real scientists, great for politicians, "parrot" media, and parents (ones that let others think and tele-raise their kids), which is what this was made for anyway. Just to help push that narrative along...

The only thing I WILL say, is that I do feel far more comfortable with people using drugs outside of their prime developmental years. You only have ONE chance to get that right and drugs, even the stuff your doctor prescribes (usually the bad stuff is in a no-win situation) can be bad. It doesn't mean MJ will hurt you, it just means there are a lot of "potentials" for it.

Non-physical addiction pain-killers for long-term use 50x more powerful than MJ. There seems to be little if ANY side effects, which the opiates have in spades. Their buyers are addicted to the opiates, so this could cost them.

Aug 28, 2012
Kochevnik, Since Progressives and Liberals encourage and even glamorize smoking POT (go to hemp fest in Seattle and ask the people what party they support), it shows that either progressives have lower IQ from the start or explains why they have low IQ.

I know this was probably a troll, if it wasn't then it was just horrible writing and thinking.

You should probably go look at ALL of those scientists out there in the world. Find out if they are Republicans and fit into your little world-view. You'll find out that you'll have a few people to point at and then it will suddenly stop.

Scientists don't like to be engaged in politics, they show-up to get their funds. But, around the world you'll find that they're closer to being Democrat or being farther left than you think of as "left", maybe progressive as well.

Sadly, whatever GREAT evidence you think you have to explain away the people whose viewpoints are different than yours, just doesn't exist. Marijuana too. (tl, s!)

Aug 28, 2012
From my understanding, IQ decreases with age, regardless. Up to 20 points.

Was this experiment done with a control? Were people who did not use marujuana also followed? Was it double-blind?

This does not seem like science to me.

Aug 28, 2012
We need to legalize access to all chemical substances while educating everyone about proper preparation for and context in which use is appropriate. Children need and deserve to experience drug-free growth and adolescence, establishing experiential normal as a baseline for voluntary departure & consciousness expansion as adults if they wish. Therefore making drugs available to children should be outlawed by society. Adults wishing to alter their states should be issued licenses for doing so, call them psychonaut licenses, much like driver's licensing for automobiles. Problem solved. Want to create a problem? Try prohibition!!!

Aug 28, 2012
I am absolutely amazed at some of the comments above. Some of you have clearly not even read the article, or do not understand it. This is a longitudinal study of two cohorts, and thus contains its own control group (i.e. the group that did not smoke prior to age 18). Almost all other factors such as education (which is specifically detailed), socio-economic status, sporting activities and health can be accounted for given the very nature of the study. Given you're making a comparison between matched groups, it even controls for IQ changes over time.

Most importantly, this fits with the massive amount of experimental and neurological evidence for continued brain development during adolescence. It is a well demonstrated fact that heavy alcohol use during early teen years has similar detrimental neurological effects. It is highly unsurprising that the same appears true for marijuana.

Dismissing what, on face value, appears a relatively sound study based on one's own bias is idiotic.

Aug 28, 2012
Additionally society needs a worldwide chain of centers in which safe exploration of chemically altered states of consciousness can happen. Adults who do not have a safe environment in which to explore chemically altered states could check in, safely explore their experiences and take their learnings back home. Counselors could be made available as well as blood chemistry and genetic monitoring could contribute to counsel which substances are not safe for any given individual. For example, alcohol is NOT SAFE for indigenous indians because of their genetic makeup... These parameters can be established for all consciousness-changing chemicals and safe medical backup made available by society would virtually eliminate "bad" trips, incarceration and social stigmas. PROBLEM SOLVED. Want to create a problem. Declare a "Drug War". Create Prohibition. Overload Prisons with people who do not belong there!!!

Sep 01, 2012
I am absolutely amazed at some of the comments above.

As I am sure many are amazed at the short-sightedness of your comments here. What if they had done a study on aspartame that is PROVEN to cause brain lesions in ANYONE. Why is it not socially acceptable to mock people who drink diet sodas that contain pesticide as a sweetener? People who say 'Never have, never will" Sure has heck should never get more than half the say in the marijuana issue. If you haven't tried it then how do you have any idea what the effects entail? Sure it may damage your cilia if smoked but eat it or vaporize it and you remove 99% of the harmful effects of marijuana. Also it has been proven to GROW braincells in your long term memory centers of your brain.

The only thing to worry about is that THC is the perfect mind control chemical, so if you smoke pot, for the love of god dont watch TV just go outside and be with nature or make something from natural stuff with your bare hands. Life is not media.

Sep 03, 2012
As I am sure many are amazed at the short-sightedness of your comments here.

Your comment nicely illustrates the completely unfounded criticism of this particular study I was concerned about. I notice you do not engage in any scientific criticism of the methods used, but rather engage in an appeal to emotion based on a completely unrelated health issue. As I previously mentioned, the study is based on a sound epidemiological method, and fits with the scientific literature about the neurological effects of many drugs during development. Additionally, it fits with data from animal models of marijuana consumption. My point is that criticism of this study is not valid simply on the basis of it being part of a 'grand conspiracy'. It's content is wholly in keeping with the scientific method and criticism should relate to that method and whether the conclusions drawn from the results are reasonable.

Why do you think I haven't tried marijuana even though I say nothing of the sort

Sep 07, 2012
Marijuana enjoyers are no more advanced intellectually than the teens who videotaped themselves putting their hands in rat traps and springing them. Normal people don't engage in the kind of recreational marijuana use Arton and such are endorsing! Those who do are "ruined", but they didn't have far to fall to failurehood to begin with.
Damn it. There goes another theory I had to explain julianpenrod's paranoid fantasies. If it's not Mary Jane, what explains julian's mental damage?

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