A teenager smokes a marijuana joint at the Vancouver Art Gallery during the annual 4/20 day, which promotes the use of marijuana, in Vancouver, British Columbia, April 20, 2013. Scientists have found a correlation between heavy marijuana use and decreased gray matter in certain parts of the brain. Reuters

Scientists have found that using marijuana heavily over many years can actually shrink the human brain, especially in people who started smoking pot in their early teens, according to a new study published Monday in the journal Proceedings of the National Academy of Sciences. Researchers studying the long-term effects of marijuana on the brain found that compared with nonusers, habitual marijuana smokers’ had less gray matter, the stuff that contains most of the brain’s nerve cells, in the area of the brain responsible for decision-making. Essentially, their orbitofrontal cortexes had shrunk, scientists suggested.

"The younger the individual started using, the more pronounced the changes," Francesca Filbey, associate professor at the School of Behavioral and Brain Sciences at the University of Texas at Dallas and the study's lead author, told CNN. "Adolescence is when the brain starts maturing and making itself more adult-like, so any exposure to toxic substances can set the course for how your brain ends up."

Marijuana is often touted for its short-term health benefits such as pain-relieving properties for chemotherapy patients. Less is known about the lasting effects of chronic marijuana use.

Researchers looked at 48 marijuana users who started smoking between the ages of 14 and 30. The average age of those involved in the research was 18, CNN reported. Scientists compared MRI scans of the smokers’ brains as well as their performances on IQ tests to those of nonsmokers. They found that subjects who had regularly used marijuana had IQs that were, on average, five points lower than people who did not smoke pot.

While study's authors could not say definitively that smoking pot led to lower IQs, MRI scans did show a reduction in gray matter in heavy smokers. “What’s unique about this work is that it combines three different MRI techniques to evaluate different brain characteristics,” Sina Aslan, founder and president of Advance MRI, LLC and adjunct assistant professor at the University of Texas at Dallas, said in a statement. “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.”