Illustration by Sydney Rae Hass for TIME
By Alexandra Sifferlin
August 27, 2015
TIME Health
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Using marijuana does not cause changes in brain volume, a new study suggests.

Public health experts have cited concerns that using marijuana could be associated with structural changes in the brain. However, a new trial comparing the brains of marijuana users and non-users to their siblings reveals that marijuana use likely does not cause changes in brain volume.

In the study, published in the journal JAMA Psychiatry, researchers looked at a large group of siblings ages 22 to 35. Of the 483 people, 262 reported ever using marijuana, even just once. The researchers then split the men and women into groups: sibling pairs who had never used marijuana, sibling pairs where both had reported using marijuana, and sibling pairs where one had used marijuana and one had not. Overall, they noticed that people who reported using marijuana had smaller volumes in certain parts of the brain—like the left amygdala, which is involved in emotional processing. However, these differences still fell within a range of volume that is considered normal.

The researchers hypothesized that in the sibling pairs where one had used marijuana and one had not, they would see differences in brain volume. But instead, they found that the exposed and unexposed siblings had the same amygdala volume. “We found no evidence for the causal influence of cannabis exposure on amygdala volume,” the authors concluded.

The researchers suggest that differences in volume could be due to other factors, like genetics or living environment. “Our study suggests that cannabis use, or at least the simple index of it that we used, does not directly impact changes in brain volumes,” says study author Arpana Agrawal, an associate professor at Washington University School of Medicine. “Instead, any relationship that we did see between cannabis use and brain volumes was due to predisposing factors that influence both cannabis use and brain volumes.”

The study did not find that brain volume has any effect on whether or not a person uses marijuana.

Another study, also published by different authors in the same journal, found that using marijuana could alter the brains of males at high risk for schizophrenia in potentially meaningful ways.

More research needs to be done to understand whether marijuana does or does not have potentially harmful effects on the brain, or whether the risks are different from one person to the next.

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