Researchers from the Center for BrainHealth at The University of Texas at Dallas have found evidence that the effects of chronic marijuana use may depend on when a person started smoking pot and for how long.

For the study, which was published in the journal Proceedings of the National Academy of Sciences on Nov. 10, Francesca Filbey from the School of Behavioral and Brain Sciences at the University of Texas at Dallas, and colleagues involved 48 adult marijuana users who started to use weed when they were between 14 and 30 years old.

The participants smoked pot thrice a day on average; most admitted that they had been using weed for 10 years. Some participants reported they had been using marijuana for 30 years.

The researchers had the participants take an IQ test and undergo MRI scans. They then compared the results with those of 62 individuals who were nonusers and found that in cognitive tests, marijuana users had lower IQ compared with those who did not smoke pot.

Filbey and colleagues noticed that marijuana users who started smoking pot when they were 14 years old had less gray matter in their orbitofrontal cortex, the area of the brain involved in decision-making. They also observed that among chronic users, there is increased brain connectivity, which is crucial for adaptive learning abilities and helps the brain make associations. This deteriorates with chronic use of marijuana.

"Marijuana users had significantly less bilateral orbitofrontal gyri volume, higher functional connectivity in the orbitofrontal cortex (OFC) network, and higher structural connectivity in tracts that innervate the OFC (forceps minor) as measured by fractional anisotropy (FA)," the researchers wrote. "Increased OFC functional connectivity in marijuana users was associated with earlier age of onset."

Chronic marijuana users scored five points lower in the IQ test in general compared with the nonusers, albeit the researchers acknowledged that it does not prove marijuana alone is responsible for the users' lower IQ.

"While our study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, these effects do suggest that these changes are related to age of onset and duration of use," Filbey said.

Several studies have questioned the findings of a 2012 Duke University research that claimed to have found an association between heavy marijuana use and cognitive decline among teenagers, but the study by Filbey and colleagues appears to complement the idea that chronic marijuana use indeed has a negative effect on the brain, particularly of young users.

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