Chronic Weed Use Shrinks Brain Region

Long-term marijuana smokers have less gray matter in their orbitofrontal cortex than nonsmokers, but other brain circuits may compensate by increasing connectivity.

Bob Grant
Bob Grant
Nov 12, 2014

WIKIMEDIA, BOGDAN GIUSCAHeavy pot smokers, especially those who began using the drug in their early teens, may experience substantive alterations in brain structure, according to researchers in Texas and New Mexico. The team of scientists used MRI to measure the brains of 48 regular marijuana users who started smoking the drug between age 14 and 30. Comparing the users’ brains to those of 62 nonsmokers, they found that the weed smokers had smaller orbitofrontal cortices, the brain region that plays a major role in decision making.

“The younger the individual started using, the more pronounced the changes,” said Francesca Filbey, the study’s principal investigator and an associate professor at the School of Behavioral and Brain Sciences at the University of Texas at Dallas, told CNN. “Adolescence is when the brain starts maturing and making itself more adult-like, so any exposure to toxic substances can set the course for how your brain ends up.” Fibley and her colleagues published the research in PNAS on Monday (November 10).

But the researchers also found that the altered brains of users seemed to compensate for orbitofronal cortex shrinkage by increases in white matter, which may improve connectivity within other brain regions. “We found that while the orbitofrontal cortex was smaller, there was greater functional and structural connectivity,” Filbey told The Washington Post. “The white matter seemed to have greater integrity [in marijuana users]. And the connection between the orbitofrontal cortex and other areas were stronger.” This compensatory effect was more prominent in users who began smoking at an earlier age and who had been smoking for a shorter duration. In smokers who had been using for about six or seven years, both connectivity and integrity began to erode.

The study, which was funded in part by the National Institute on Drug Abuse, is the latest in a slate of recent papers that come to differing conclusions about the dangers (or lack thereof) of marijuana use. And Fibley and her coauthors acknowledge that their results aren’t conclusive. A smaller orbitofrontal cortex, for example, may lead a person to use marijuana rather than being a result of years of smoking.