Young marijuana smokers showed significant structural abnormalities in two key areas of the brain — the “pleasure center” and a brain region central to emotional response, a small study has found.
The study compared magnetic-resonance images of the two brain regions of 40 Boston-area youths ages 18-25. Half of them smoked an average of about 11 joints a week over a 90-day period; the other half, no marijuana.
Other studies have focused on brain changes of longtime, heavy marijuana users.
But the study’s lead author, Jodi Gilman, a Harvard Medical School psychology instructor and a neuroscientist at Massachusetts General Hospital, said this is the first research that scrutinized changes to the brain in young, casual users of marijuana.
“I think that the reason why this study is novel is that we weren’t looking at these long-term, habitual, heavy users who had been using for years and years and years,” Gilman said.
“Some of [the marijuana smokers in the study] used only once or twice a week, and they were young, 18 to 25. Some of them had been smoking a few years. We’re already seeing these brain changes.”
The study, published Wednesday in The Journal of Neuroscience, found a direct relationship between the amount of marijuana smoked and the extent of changes in brain structure.
In the marijuana smokers, the nucleus accumbens — the pleasure center known to be involved in reward processing and addiction — was larger, abnormal in shape and denser compared with that of non-users. Among pot smokers, the shape and density of the amygdala — a region of the brain that has been shown to process emotions — were altered compared with that brain region among non-users.
Marijuana users in the study had an average age of onset of use of the drug of about 16 ½ years old and had been smoking pot an average of about six years.
But none of the users met criteria for marijuana dependency¸ determined by factors such as problems with work or school, withdrawal symptoms or increased tolerance to the drug.
“Even the heaviest of this group [marijuana users] are still considered recreational users because they’re not experiencing addiction symptoms,” Gilman said.
“So these participants, they thought they were fine, but even in these participants, we saw these brain differences, which I thought was really interesting because that indicates that these brain changes might occur very early on.”
Gilman said more research would be needed to replicate the findings of the small study.
“If these findings hold up,” she said, “I think they’re very striking because currently, we don’t know how much marijuana is safe, and this study indicates that there are observable changes in brain structure with marijuana even in these young, recreational users.”
Gilman also said more research would be required to determine the relationship between the structural changes in the brain and behavior and functions like memory, decision-making and motivation as well as how the brain changes relate to who becomes addicted.
She noted the National Institute on Drug Abuse estimates 9 percent of recreational users of marijuana go on to become problem users.
Researchers say the brain continues to develop through adolescence until about age 25.
“The brain is still developing, and it may be more vulnerable to cannabis-induced changes than an adult brain,” Gilman said.
“But the other thing to keep in mind, is this is an age when people are making all kinds of important decisions about their lives that will affect the rest of their lives, and you don’t want your decision-making capacity to be compromised … I mean this is when you’re choosing a major, you’re starting a career, you’re forming relationships. I think that if your decision-making capability is compromised, it may have ramifications for the rest of your life.”
It’s also an age when many people start using marijuana, she said.
Carl Lupica, who studies drug abuse at the National Institute on Drug Abuse, said previous studies have “largely ignored the brains of casual [marijuana] users.”
In a news release issued by the Society for Neuroscience, the publisher of The Journal of Neuroscience said, “This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy.”
The study was funded by the National Institute on Drug Abuse, the Office of National Drug Control Policy’s Counterdrug Technology Assessment Center and the National Institute of Neurological Disorders and Stroke, the release said.
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