Center for BrainHealth Study Examines Marijuana’s Long-Term Effects

The effects of chronic marijuana use on the brain may depend on when the user first consumed the drug and the duration of use, according to new research from the Center for BrainHealth at UT Dallas. 

In a paper published Monday in Proceedings of the National Academy of Sciences, researchers found that chronic marijuana users have smaller brain volume in the orbitofrontal cortex, which is commonly associated with addiction, and increased brain connectivity. It’s the first time researchers have described chronic users’ brain abnormalities with multiple magnetic resonance imaging (MRI) techniques. 

“To date, existing studies on the long-term effects of marijuana on brain structures have been largely inconclusive due to limitations in methodologies,” said Dr. Francesca Filbey, associate professor in the School of Behavioral and Brain Sciences at UT Dallas and director of the Cognitive Neuroscience Research in Addictive Disorders at the Center for BrainHealth. 

“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.” 

The research team studied 48 adult marijuana users and 62 nonusers, accounting for potential biases such as gender, age and ethnicity. Tobacco and alcohol use also were controlled in the study. On average, the marijuana users in the study consumed the drug three times per day. 

Cognitive tests show that the users had lower IQ than nonusers, but the differences appear unrelated to the brain abnormalities as no direct correlation can be made between the decreases in IQ and the volume of the orbitofrontal cortex. 

Earlier onset of regular marijuana use induces greater structural and functional brain connectivity, tests found. The greatest connectivity increases appear as an individual begins using marijuana. Findings also show that severity of use is directly correlated to greater connectivity. 

“What’s unique about this work is that it combines three different MRI techniques to evaluate different brain characteristics,” said Dr. Sina Aslan, founder and president of Advance MRI, LLC and adjunct assistant professor at UT Dallas. “The results suggest increases in connectivity, both structural and functional, that may be compensating for gray matter losses. Eventually, however, the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.” 

Although wiring declines after six to eight years of continued chronic consumption, marijuana users continue to show more intense connectivity than nonusers. That may explain why they “seem to be doing just fine” despite smaller brain volumes in the orbitofrontal cortex, Filbey said. 

The study also indicates that gray matter in the cortex may be more vulnerable than white matter to the effects of delta-9-tetrahydrocannabinol (THC), the main psychoactive ingredient in the cannabis plant. According to the researchers, chronic marijuana use initiates a complex process that allows neurons to compensate for smaller gray matter volume.

But further studies are needed to determine whether discontinued marijuana use would reverse the effects, whether similar effects occur in occasional marijuana users and whether they are a direct result of marijuana use or a predisposing factor.

“We have seen a steady increase in the incidence of marijuana use since 2007,”  Filbey said. “However, research on its long-term effects remains scarce despite the changes in legislation surrounding marijuana and the continuing conversation surrounding this relevant public health topic.”

The research was funded by the National Institute on Drug Abuse.