Chronic Pot Smoking Associated With Reduced Gray Matter, But Increased Connections

guest author image

Justine Alford

Guest Author

196 Chronic Pot Smoking Associated With Reduced Gray Matter, But Increased Connections
Torben Hansen, via Flickr. CC BY 2.0

Another study has found that marijuana use is associated with changes in the brain. The study compared the brains of chronic users with non-users and discovered the former had less volume in a region that is involved in decision making and emotional processing. However, they also found that this region showed increased connections with other brain regions in users, suggesting that the brain could be re-wiring itself to make up for the shrinkage.

“The changes in connectivity may be considered a way of compensating for the reduction in volume,” said study author Francesca Filbey. “This may explain why chronic users appear to be doing fine, even though an important region of their brain is smaller in terms of volume.”


Studies investigating the potential effects of smoking pot on the brain often produce conflicting results, which is probably due to the use of different methodologies. To address this issue, researchers from the University of Texas at Dallas used multiple magnetic resonance imaging (MRI) techniques to build a comprehensive picture of the brains of almost 50 marijuana users alongside 62 non-users that were age- and sex-matched. They also controlled for tobacco and alcohol use.

The marijuana smokers were described as chronic users, consuming the drug three times a day on average. The participants had a wide age range and also started using the drug at different ages, which allowed the researchers to characterize changes across lifespan without developmental biases.

As described in PNAS, the researchers found that, compared with controls, marijuana smokers had less volume in a brain region called the orbitofrontal cortex (OFC), which is involved in decision making, emotion and controlling reward- and punishment-related behavior. However, they also found that this region had more structural and functional connections with other brain regions, which they speculate could be an adaptive response to the reduction in OFC volume.

They also discovered that the earlier the participants started using marijuana, the greater the increase in brain connectivity in the OFC. Furthermore, the biggest increases in connectivity were found to appear as participants begun using marijuana. However, the re-wiring was found to decrease after 6 to 8 years of chronic use, although users still continue to display more connections than non-users.


Although the researchers found that, on average, users had lower IQ compared to matched controls, the differences were not related to the observed brain changes as no correlations existed between IQ and OFC volume.

Marijuana advocates have also called into question the usefulness of this study as it failed to assess whether the observed changes were associated with adverse performance outcomes, such as mental abilities or quality of life.

“It may be that these cannabis users are functioning in their daily lives in a manner that is indistinguishable from controls,” added Paul Armentano of the National Organization for the Reform of Marijuana Laws, “in which case these imaging differences may hold little, if any, real-world significance.”

Furthermore, the researchers accept that the study cannot determine whether smoking marijuana caused the brain changes or vice versa, and call for long-term studies to look into this. 


[Via PNAS, WebMD and UT Dallas]


  • tag
  • Marijuana,

  • Cannabis,

  • MRI