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Marijuana use can have an impact on the brain, a new study reported. The researchers added that the extent of the effects depends on when people started using marijuana and for how long.

"We have seen a steady increase in the incidence of marijuana use since 2007," said Dr. Francesca Filbey, Director of the Cognitive Neuroscience Research in Addictive Disorders at the Center for BrainHealth and Associate Professor in the School of Behavioral and Brain Sciences at The University of Texas at Dallas. "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."

For this study, the researchers recruited 48 adults with a history of marijuana use and 62 adults who were non-users. The users reported taking the drug an average of three times per day. All of the participants underwent cognitive tests and multiple magnetic resonance imaging (MRI) scans.

After controlling for age, gender, ethnicity, and tobacco and alcohol use, the researchers found that adults who used marijuana tended to have lower IQs (intelligence quotient), which measures intelligence. Users were more likely to have a smaller brain volume in the part of the brain linked to addiction known as the orbitofrontal cortex (OFC). They also had increased brain connectivity. Despite the differences observed between users and non-users, the researchers noted that a low IQ was not directly linked to a smaller OFC.

"What's unique about this work is that it combines three different MRI techniques to evaluate different brain characteristics," Dr. Sina Aslan, founder and president of Advance MRI, LLC and adjunct assistant professor at The University of Texas at Dallas, commented reported in the press release. "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."

The team added that adults who started using marijuana at an earlier age were more likely to experience structural and functional connectivity. The increased structural wiring in the brain linked to marijuana use started to decline after about six to eights years of chronic marijuana use. At this point, the researchers found that the chronic users' brains were still more intensely connected in comparison to the brains of non-users.

"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. Filbey. "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 study was published in the Proceedings of the National Academy of Sciences (PNAS).