Those who smoke marijuana casually are not immune to the structural brain changes that can affect heavy users, according to a study published today in the Journal of Neuroscience.

Researchers from Northwestern University Feinberg School of Medicine, Harvard Medical School, and Massachusetts General Hospital used MRI scans to compare the brains of 20 18- to 25-year-olds who smoked at least once a week to those of 20 individuals who had little or no history of using the drug. Though the low to moderate marijuana users were not addicted to the drug, imaging data showed that their brain anatomy had changed.

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The More You Smoke, the More Your Brain Changes

The nucleus accumbens, a part of the brain linked to reward processing, was larger and had changed shape in casual smokers compared to non-users. The researchers also found that the amygdala, which helps to regulate emotions, had also changed in shape and density in those who smoked the drug. The more marijuana users said they ingested, the more abnormalities the scientists detected on the brain scans.

“While I don't think anyone has directly contrasted recreational with dependent users, it is pretty clear from our data that the more you use, the more the brain is impacted,” said Anne Blood, Ph.D., who leads the Laboratory for Mood and Movement Disorders at Massachusetts General Hospital.

“This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences,” added Dr. Hans Breiter, a professor of psychiatry and behavioral sciences at Northwestern University, who was also involved with the study.

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'No Doubt' About the Impact of Casual Use

Blood said that any brain changes have the potential to cause behavioral and neurological changes as well. The parts of the brain where they detected structural differences are “powerhouses of function.”

“These are not brain regions that you want to alter,” she said, calling for more studies that target recreational marijuana users. “There is no doubt that we will find these structural changes have some impact on these individuals' neurological or psychiatric and behavioral function.”

According to data from the National Survey on Drug Use and Mental Health (NSDUH), 18.9 million Americans say they've recently used marijuana. The drug is commonly linked to problems with attention, learning, and motivation. Previous studies involving animal exposure to tetrahydrocannabinol (THC)—the main psychoactive ingredient in the drug—have shown that repeated exposure can alter the brain. Until now, there was little research on low-level exposure to THC.

“This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy,” Carl Lupica, Ph.D., who studies drug addiction at the National Institute on Drug Abuse, and who was not involved with this study, said in a statement. “These observations are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users.”

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