Does using marijuana shrink the brain?

New research on marijuana's impact on personal health shows that it's not just bank accounts that can shrink due to long-term recreational use. The drug may also decrease brain volume.

“Long-term effects of marijuana use on the brain,” (paywall) published this week by Proceedings of the National Academy of Sciences, showed that regular marijuana users had less gray matter in their orbital frontal cortex than nonusers. That region of the brain “is a key node in the brain's reward, motivation, decision-making and addictive behaviors network,” The Los Angeles Times reported.

The study highlighted how the age at which marijuana use begins determines the extent of the changes in the brain. “The younger the individual started using, the more pronounced the changes,” explained Francesca Filbey, the study's principal investigator, to CNN.

She added, “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.”

It wasn't all bad news for regular marijuana users, however, The New York Times reported. The MRI scans of chronic users, defined as people who use the drug an average of three times per day, showed that their brains had adapted to make up for the loss of volume.

“There was evidence that the brain compensated for this loss of volume by increasing connectivity and the structural integrity of the brain tissue,” the New York Times reported.

Although the study was applauded for discovering a connection between regular marijuana use and altered brain structures, it still left some questions unanswered, the LA Times noted, as is often the case in marijuana research.

“The authors of the study acknowledge that they cannot discern whether a pot smoker's smaller orbital frontal cortex is the cause or the result of chronic marijuana use,” the LA Times reported.

This inability to parse cause and effect has plagued researchers as they investigate the short- and long-term health consequences of recreational marijuana use, Scientific American reported last year. “Although many physicians agree that marijuana is safe enough to temporarily alleviate the symptoms of certain medical conditions, the safety of recreational use is poorly understood.”

The Proceedings of the National Academy of Sciences study compared urine samples, MRI scans and IQ tests from 48 marijuana users and 62 nonusers, CNN noted. The users had all started their drug habit between age 14 and 30 and “most said they had been using (marijuana) for 10 years.”

Email: kdallas@deseretnews.com Twitter: @kelsey_dallas