Young adults who used marijuana only recreationally showed significant abnormalities in two key brain regions that are important in emotion and motivation, scientists report.

The study was a collaboration between Northwestern Medicine and Massachusetts General Hospital/Harvard Medical School.

This is the first study to show casual use of marijuana is related to major brain changes.

It showed the degree of brain abnormalities in these regions is directly related to the number of joints a person smoked per week. The more joints a person smoked, the more abnormal the shape, volume and density of the brain regions.

“This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” said corresponding and co-senior study author Hans Breiter, M.D.

He is a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and a psychiatrist at Northwestern Memorial Hospital.

“Some of these people only used marijuana to get high once or twice a week,” Breiter said. “People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.” 

The study was published today in the Journal of Neuroscience.

Scientists examined the nucleus accumbens and the amygdala — key regions for emotion and motivation, and associated with addiction — in the brains of casual marijuana users and non-users. Researchers analyzed three measures: volume, shape and density of grey matter (i.e., where most cells are located in brain tissue) to obtain a comprehensive view of how each region was affected.

Both these regions in recreational pot users were abnormally altered for at least two of these structural measures. The degree of those alterations was directly related to how much marijuana the subjects used.

Of particular note, the nucleus acccumbens was abnormally large, and its alteration in size, shape and density was directly related to how many joints an individual smoked. 

“One unique strength of this study is that we looked at the nucleus accumbens in three different ways to get a detailed and consistent picture of the problem,” said lead author Jodi Gilman, a researcher in the Massachusetts General Center for Addiction Medicine and an instructor in psychology at Harvard Medical School. “It allows a more nuanced picture of the results.” 

Examining the three different measures also was important because no single measure is the gold standard. Some abnormalities may be more detectable using one type of neuroimaging analysis method than another. Breiter said the three measures provide a multidimensional view when integrated together for evaluating the effects of marijuana on the brain.  

“These are core, fundamental structures of the brain,” said co-senior study author Anne Blood, director of the Mood and Motor Control Laboratory at Massachusetts General and assistant professor of psychiatry at Harvard Medical School. “They form the basis for how you assess positive and negative features about things in the environment and make decisions about them.”

Through different methods of neuroimaging, scientists examined the brains of young adults, ages 18 to 25, from Boston-area colleges; 20 who smoked marijuana and 20 who didn’t. Each group had nine males and 11 females. The users underwent a psychiatric interview to confirm they were not dependent on marijuana. They did not meet criteria for abuse of any other illegal drugs during their lifetime.

The changes in brain structures indicate the marijuana users’ brains are adapting to low-level exposure to marijuana, the scientists said.

The study results fit with animal studies that show when rats are given tetrahydrocannabinol (THC) their brains rewire and form many new connections. THC is the mind-altering ingredient found in marijuana.

“It may be that we’re seeing a type of drug learning in the brain,” Gilman said. “We think when people are in the process of becoming addicted, their brains form these new connections.”

In animals, these new connections indicate the brain is adapting to the unnatural level of reward and stimulation from marijuana. These connections make other natural rewards less satisfying.

 “Drugs of abuse can cause more dopamine release than natural rewards like food, sex and social interaction,” Gilman said. “In those you also get a burst of dopamine but not as much as in many drugs of abuse. That is why drugs take on so much salience, and everything else loses its importance.”

The brain changes suggest that structural changes to the brain are an important early result of casual drug use, Breiter said. “Further work, including longitudinal studies, is needed to determine if these findings can be linked to animal studies showing marijuana can be a gateway drug for stronger substances,” he noted.

Because the study was retrospective, researchers did not know the THC content of the marijuana, which can range from 5 to 9 percent or even higher in the currently available drug. The THC content is much higher today than the marijuana during the 1960s and 1970s, which was often about 1 to 3 percent, Gilman said. 

Marijuana is the most commonly used illicit drug in the U.S. with an estimated 15.2 million users, the study reports, based on the National Survey on Drug Use and Health in 2008. The drug’s use is increasing among adolescents and young adults, partially due to society’s changing beliefs about cannabis use and its legal status.

A recent Northwestern study showed chronic use of marijuana was linked to brain abnormalities. “With the findings of these two papers,” Breiter said, “I’ve developed a severe worry about whether we should be allowing anybody under age 30 to use pot unless they have a terminal illness and need it for pain.”

The research was supported by grants 14118, 026002, 35 026104, 027804 and 034093 from the National Institute on Drug Abuse, and grant 052368 from the National Institute of Neurological Disorders and Stroke, all of the National Institutes of Health. The Office of National Drug Control Policy and Northwestern Medicine’s Warren Wright Adolescent Center also supported the research.

Join the Conversation


  1. City government cares more about money than kids

    With this new scientific evidence do you think the Evanston City Council would reconsider its 2011 ordinance to decriminalize marijuana?

    ANYONE caught with 10 grams of pot or less would get a ticket rather than be arrested. The Council and mayor said they did it to "keep a young person’s criminal record from hindering future employment." The only problem is there is not an age limit on the ordinance so an "old person"  caught with less than 10 grams of pot would not get a record. 

    The real reason for the pot ordinance was not to help kids but to get more revenue for the city. If the Council and mayor care about kids they would repeal the city's pot ordinance. But the government's greedy hand cares more about revenue and political power than our kids. 

    1. Rewire your amygdala

      City and state should decriminalize marajuana for everyone over the age of 18.

      Marijuana enlarges the nucleus accumbens and the amygdala…so it really does expand your mind. The article does not show that these alterations are harmful, it only shows that there is a causal connection between brain alteration and marijuana use.  

      "The study results fit with animal studies that show when rats are given tetrahydrocannabinol (THC) their brains rewire and form many new connections. THC is the mind-altering ingredient found in marijuana." 

      The study may just as well turn out to support the use of marijuana as a "gateway" to social and political change.


  2. Why is this story on EvanstonNow?

    I first saw this story on Outside of the brain "abnormalities", no other symptoms/ill effects were reported by the subjects, nor observed by the researchers.

    The previous comment makes a lame attempt to link this study to Evanston's decision to decriminalize possession of small amounts of marijuana, as if this one small study justifies bringing the full weight of the judicial system down upon some unfortunate soul caught with a joint.

    Reminds me of that old anti-drug commercial of "this is your brain" (an egg) and "this is your brain on drugs" (frying pan with sunny-side egg.)

    1. We don’t care about no stinkin brain abnormalities

      Outside of brain abnormalities there were no other ill effects reported by the subject, according to CNN.

      Right. So brain abnormalities is no big deal to you John. What's a little brain abnormality between friends? Eh, John?

      It's amazing CNN actually did some other news coverage aside from the Malaysian plane that went missing March 7. Was that when Fareed Zakaria was for recreational pot use before he was against it?  

      Possession of marijuana in Illinois is illegal. Period. Enforcing the law by arresting someone with a small amount of pot is hardly bringing the full weight of the judicial system down on someone.

      BTW – 10 grams equals roughly 20 joints.Put it another way, dealers sell marijuana in quarters, which is 7 grams that nets about $100 on the street. The anti-drug commercial of "this is your brain" (an egg) and "this is your brain on drugs" (frying pan with sunny-side egg) was about drugs not just marijuana.

      And why is this story on EvanstonNow, asks Zbesko. Perhaps because the study was done in part by Northwestern Medicine and the story is a Northwestern News press release. Last I checked Northwestern is located in Evanston and EvanstonNow covers Evanston News.



  3. Balderdash!

    The use of only twenty subjects does not make this a valid scientific study. The sample size is simply too small to draw any significant results, whatsoever.

    At best, this study is a call for more work on the effect of THC on brain chemistry. At worst, it's alarmist junk-science.


    1. Scientist Blasts Report

      Scientist Blasts Report Linking Casual Pot Smoking With Brain Abnormalities

      "The hype surrounding this manuscript is unwarranted," Keller said. "The sample size is woefully small. The statistical methods used — to the extent that they can be assessed from the very brief descriptions in the manuscript — are inappropriate. Even if the study was done appropriately, the results do not reach statistical significance."


      Ah, yes, the hype. Antagonize those righties and lefties. Drive up those page views. Collect the ad fees.

      1. Can’t change facts just cuz you don’t like the study

        Ah yes, two scientists disagree with the study and therefore it must be overhyped.

        Yet, another scientists in your link said the study was valid. Consider that every study must pass the rigors of review from the board of the Journal of Neuroscience before it can be published. Not any junk study will be published in this prestigious journal.

        Just because you don't like the results of the study doesn't mean you should accept it as invalid.

        Common sense should tell ya that recreational pot use is unhealthy and harmful.


        1. Did they have a bias or non-scientific reason ?

          You can always find those who are paid by the industry–see decades of tobacco "research" by the industry or paid consultants.  You also find those who want to make a name for themselves or hope to become paid consultants—we know what they are worth.

          I'm waiting for the lawsuits over Evanston's "law" [read "blind eye"] by State/Federal government—$$$$ in court costs.  Or when a parent sues the city when a child is in an accident, hurts someone or fails in school—again $$$ that the taxpayers will payout since the Council does not bankroll legal costs out of "their" pocket.

        2. Can’t trust the Supreme Court

          Anonymous Al, you might want to consider this study:

          You Can’t Trust the Supreme Court, and Now There’s Scientific Proof

          "A recent study adds to the growing evidence that our brains reject information that rebuts our strongly held beliefs. Dartmouth political scientist Brendan Nyhan and three co-authors presented parents with various messages intended to encourage them to vaccinate their children. What they found, however, was that “[n]one of the interventions increased parental intent to vaccinate a future child,” and, among the parents who were most likely to be skeptical of vaccination, the messages actually backfired. Staunch deniers of the health benefits of vaccination actually said they were less likely to vaccinate their children after being presented with information supporting vaccination.

          As science journalist Chris Mooney explains, this is not an isolated study. Similar effects have been demonstrated when conservatives are presented with information debunking a common conservative misconception regarding tax policy. Or when fans of former vice presidential candidate Sarah Palin are presented with information debunking her claim that the Affordable Care Act authorizes “death panels.” Or when staunch opponents of President Obama are presented with information debunking the claim that he is a Muslim.

          Liberals should not look at these studies and become smug, however, as other research has shown that progressives are no less susceptible to motivated reasoning."

          So, I guess I'm susceptible, too, but the first step in overcoming this effect is acknowledging that it exists.

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