The Brain on (Lots of) Marijuana

It's too early to draw conclusions from a study that shows changes in the brains of pot smokers.

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Marijuana's effect on the brain is far from understood, but Australian research published Monday in the Archives of General Psychiatry suggests that very heavy long-term smoking might be associated with structural changes in two areas of the brain rich in receptors to the drug. The hippocampus, believed to regulate emotion and memory, and the amygdala, which plays a role in aggression and fear, were smaller—12 percent and 7 percent, respectively—in a group that smoked at least five joints daily for at least the past 10 years (and, on average, 20 years) when compared to a nonsmoking group.

Users also showed more signs of sub-threshold psychotic symptoms compared with those in the group that abstained. And on tests of memory and verbal ability, they performed more poorly. "Our findings suggest that everyone is vulnerable to potential changes in the brain, some memory problems, and psychiatric symptoms if they use heavily enough and for long enough," says lead author Murat Yucel of the ORYGEN Research Centre and Melbourne Neuropsychiatry Centre at the University of Melbourne. Pot has been in the news lately for other reasons, too: a government report on a possible connection between pot smoking and depression and also the possible link between heart disease risk and marijuana use.

But it's way too early for parents to conclude that pot deteriorates the brain, cautions Scott Swartzwelder, professor of psychiatry and behavioral sciences at Duke University whose own research focuses on substance abuse and the adolescent brain. "Scientifically, it's a very limited set of data," he says. The study was tiny—it covered only 15 pot smokers and 16 abstainers—and looked at extreme behavior, so "I'm not sure how relevant it is to the general public," says Swartzwelder, who is coauthor of Just Say Know: Talking to Kids About Drugs and Alcohol and Buzzed: The Straight Facts About the Most Used and Abused Drugs From Alcohol to Ecstasy (an updated third edition is being released in August). An earlier U.S. News story looked at some of the science on pot and how it relates to the developing brain.

Yucel acknowledges that the size of the group is an issue, noting the difficulty of finding subjects who smoked a lot of pot but didn't also do other drugs or have medical or psychological issues. Another unanswered question, says Swartzwelder, is the importance of the size of a person's hippocampus and amygdala. "It's tempting to say smaller is worse, but that's a trap. You don't know with any degree of certainty that these pot smokers didn't have smaller brain structures to begin with—maybe they have smaller hippocampus and amygdala, and that's what motivates them to smoke pot in the first place."

An important unaddressed question from parents' point of view is whether the brain differences were a result of how long the men had smoked or how young they were when they began smoking regularly. "We know the younger brain is still maturing and therefore generally more susceptible to the harmful effects of drugs," Yucel says. Emerging research about marijuana, says Swartzwelder, suggests that the drug may have far more powerful effects on the teenage brain than on that of an adult.