Long-term marijuana use affects the brain, but the exact effects may depend on duration of use and the age when cannabis was first consumed, according to researchers from The University of Texas at Dallas' Center for BrainHealth.

In a study published in the journal Proceedings of the National Academy of Sciences, Francesca Filbey and colleagues comprehensively describe for the first time brain structure and function abnormalities arising from long-term marijuana use.

Utilizing several MRI techniques, the researchers determined that long-term marijuana users tend to have smaller orbitofrontal cortices, the region of the brain usually connected to addiction. However, they also exhibited an increased connectivity in the brain. And the heavier the marijuana usage, the greater these brain connections were.

"The results suggest increases in connectivity, both structural and functional that may be compensating for gray matter losses," said Sina Aslan, one of the authors of the study.

The researchers also observed that structural connectivity in the brain starts to degrade after marijuana use was continued for between six and eight years, but these brain connections in users are more intense than what non-users have. This might be the reason why long-term marijuana users appear to be fine despite the reduced volume in their orbitofrontal cortices.

For the study, the researchers worked 48 users who consumed marijuana three times a day on average and 62 age- and gender-matched non-users. Aside from accounting for biases in race, age and gender, they also controlled for alcohol and tobacco use.

When the researchers carried out cognitive tests, they saw that long-term marijuana users recorded lower IQ scores compared to the control group, although the difference doesn't appear to be connected to the observed brain abnormalities in the users.

While the study was not able to conclusively say if brain changes observed are all direct results of using marijuana, it does offer preliminary proof that the orbitofrontal cortex's gray matter may have a higher vulnerability to the effects of delta-9-tetrahydrocannabinol (THC) than white matter. THC is the primary psychoactive component in marijuana.

For future studies, the researchers suggest determining if the brain changes observed would revert back to normal when marijuana use was discontinued, if the changes will manifest as well in occasional users and if the changes were a direct consequence of using marijuana or another factor.

In another study published in the journal Obstetrics & Gynecology, researchers found that marijuana use during pregnancy doesn't increase risks for miscarriages, low birth weights and preterm births when not mixed with tobacco use.

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