Repeated marijuana use is correlated with lower IQ scores and less volume in the region of the brain that helps make decisions, according to a new study.

The study found that the average marijuana user’s IQ was about five points lower than that of a non-user. The earlier the study participants began consuming the drug, the worse the condition of the brain. The study, which compared almost 50 marijuana users to a control group, suggests that at first brains affected by marijuana compensate for the deficit in decision-making brain volume by increasing connectivity, a key brain function. But marijuana-affected brains can’t keep up in the long term.

“The results suggest increases in connectivity, both structural and functional that may be compensating for gray matter losses,” said study co-author Sina Aslan, a faculty member at The University of Texas at Dallas. “Eventually, however, the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.”

While previous studies have showed that marijuana causes harm to the brains of animals, researchers said they couldn’t be sure whether marijuana use was the cause of the negative changes in the brain. Nonetheless, the study joins a growing body of evidence that marijuana harms the brains of young people.

Write to Justin Worland at justin.worland@time.com.