Study Says Regular Pot Habit Changes Brain, May Lower IQ

This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

(CNN)- Using marijuana at an early age could have long-term consequences on your brain and it may even lower your IQ, according to a new study in the Proceedings of the National Academy of Sciences.

Researchers found that compared to nonusers, people who smoked marijuana starting as early as age 14 have less brain volume, or gray matter, in the orbitofrontal cortex. That’s the area in the front of your brain that helps you make decisions.

“The younger the individual started using, the more pronounced the changes,” said Dr. Francesca Filbey, the study’s principal investigator and associate professor at the School of Behavioral and Brain Sciences at the University of Texas at Dallas. “Adolescence is when the brain starts maturing and making itself more adult-like, so any exposure to toxic substances can set the course for how your brain ends up.”

Researchers also found increased brain connectivity in chronic users. Connectivity, when different parts of the brain connect to each other, is important for adaptive learning abilities. It also helps your mind make associations. This wiring of the brain starts to deteriorate with chronic marijuana use.

“Too much or too little of anything isn’t good. There needs to be an equal balance,” said Filbey.

There were 48 marijuana users enrolled in this study. All started smoking between age 14 and 30. The average age of the person enrolled in the study was 18. On average they used marijuana three times a day. Most said they had been using it for 10 years, although some had been smoking pot for three decades.

Scientists compared this group to a group of 62 nonusers of the same basic age and gender. All gave urine samples. All had an MRI scan and all went through IQ testing.

Filbey said the people who regularly used marijuana had IQ’s that were five points lower, on average, than the nonusers in the study, although there is no definitive proof that marijuana alone was to blame for the lower IQ.

“While our study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, these effects do suggest that these changes are related to age of onset and duration of use,” Filbey said.

Dr. Susan Weiss, associate director for Scientific Affairs at the National Institute on Drug Abuse said the study provides more strong evidence about the dangers of marijuana.

“This is a complex and interesting study that adds to the growing body of evidence that heavy marijuana use, particularly at a young age, is linked to significant adverse brain changes,” said Weiss. “This study showed that the orbitofrontal cortex, an area involved in reward, decision making and motivation, was smaller in heavy users and that other brain circuits were enhanced, likely to compensate for the diminished function in that region. Further prospective studies are needed to clarify this, but these mounting scientific findings certainly challenge the widespread belief that marijuana is a harmless drug.”

The National Institute on Drug Abuse helped fund the study.

Mason Tvert, director of communications for the Marijuana Policy Project, said marijuana use does not cause IQ loss.

“Once again, researchers have failed to find any conclusive evidence that marijuana use causes mental health problems. The researchers note their findings are non-conclusive, that they might be skewed by other factors, and that effects, if any, could be temporary, Tvert said. “The study doesn’t justify keeping marijuana illegal, nor does it say anything about making it legal. There remains no doubt that marijuana is far less harmful than alcohol to the brain and to the rest of the body. The possibility that marijuana might have some harm for some people — but might not — is not a good reason to keep arresting and punishing hundreds of thousands of adults simply for using it.”

Filbey next plans to do long-term observational studies with people who do not use marijuana to see if there are underlying factors like pre-existing conditions before exposure to THC, the psychoactive ingredient in cannabis, that could account for some of these effects.


  • Lucy

    Bullcrap. It helps focus, how could it lower IQ? I think most of these studies are false. I didn’t have half the issues I have now with focus now that I no longer use.

    • FEDUP

      Helps focus? How? My university educated associate uses the stuff on a regular basis, and he can’t focus on anything very long. Basically, it’s ruining his career.

  • Velda

    Every long term pot smoker that I know is a bit on the slow side.

    Also chronic pot heads tend to not be able to communicate very well.

    They typically are bad listeners as well, at least this has been my personal experience.

    • Leten Uno

      So you know all pot smokers and have a scientifically valid sample.
      Do you know the smart versus dumb, who is or who isn’t a pot smoke.
      Can you say for certain the smartest people you’ve ever meet were not pot smokers.
      Steven Jobs (apple) said taking ACID (harmful) was one of the most important things in my life. You call him a loser.
      Willie Nelson has almost 70 albums that have sold 40 million copies. He’s a bad communicator ?.
      Did you ever read the story of the Home Brew Computer club, the guys that invented the modern desktop computer. They thrived on the creativity MJ gave them.

      Velda I think maybe the only people you recognized may have been “stoners”. Were they on or taken any other drugs when you knew them. Did you know that the one benefit that is commonly reported with MJ usage is ‘Focus’, listening would come into that category huh?

      Velda, I wonder now if you are a toker ? You seem a little slow about reality, or maybe your just not communicating well. Or did you not listen to fact versus fiction when the now 5 states legalized and 29 others allow medicinal.

  • malcolmkyle

    “The conclusions were modest in the paper — we never say marijuana causes these changes. The media may have given that impression in headlines, but the study doesn’t show causation. I think I saw one headline that was ‘Marijuana reshapes the brain’ and I groaned — that’s not what we did,”

    ——Dr. Jodi Gilman, 31, author of the Harvard Medical School/Massachusetts General Hospital study on marijuana’s effects, in an interview with PolicyMic.  

  • atc8824

    If this is true then what is wrong with people who do not do drugs?I know several people with low IQ’s that do not do drugs.I was taught in school that alcohol kills brain cells but I do not see a study on alcohol.Why?To many people drink and wanna keep it legal.If you drink alcohol or take prescription pills such as Xanax,loricet,prozac,nyquil or any other mind altering drug then you have no right to judge pot.Drugs are drugs legal or not.Funny how people with prescription drugs think they do no wrong and how about the people with scripts that sell their scripts for money?Isn’t that just as wrong or do you people really think it is alright because they legally obtain the drug?Do not forget alcohol,pills and some over the counter drugs are still drugs.Just because you can get them legally doesn’t make it alright to abuse them.

  • Jose Gonzales

    The truth is alcohol makes the brain swollen, angry, and with a tendency to want to go to war.

    Marijuana reduces swelling, makes the brain happy, and creates a tendency to prefer peace.

    Nixon knew this and how it affected public opinion on the Vietnam War.

    IQ is a measurement of clerical skills, not creativity. Clerical skills decrease while on marijuana but quickly return after the effects where off in an hour or two. While the clerical skills are decreased, other skills increase.

  • Jonathan McCall

    Nothing in the report or the article says that regular pot habit changes brain.

    Per the article, it suggests that a few on average may have had slightly lower IQ, but was there evidence that the individuals’ IQs were lower before beginning the tests? What about the others? Larger numbers means more possibility for selection of high IQs thereby raising the IQ average of the non-smokers. Let alone, did the testers review the family history of the individuals of the people with lower IQs as well as daily habits? If the users are not staying active with mental exercises or are older, then they may not have the same scores as a college student or high school student who is actively using their brain daily for complex functions.

    Also, of note, is the blatant misconceptions in the article:
    “Researchers also found increased brain connectivity in chronic users. Connectivity, when different parts of the brain connect to each other, is important for adaptive learning abilities. It also helps your mind make associations. This wiring of the brain starts to deteriorate with chronic marijuana use.”

    If the connectivity is better with users, then how is it deteriorating?

    To me, this article appears to mislead the readers by the people or person who wrote this article in order to innately persuade readers to their ideas on the issue.

    • arnold fudpucker

      Nice argument but it doesn’t prove the drug is harmless either. Most people just know what they see so much of and that is the obvious evidence of the conditions of the users. I am going to presume you are a user and I can appreciate your trying to justify yourself but you just can’t argue away the ill affects.

Comments are closed.