Since the sizzling fried egg in the iconic 1987 Partnership for a Drug-Free America ad, we’ve been asked to think about what a “brain on drugs” looks like. Now, we have a slightly better idea, thanks to a study that used MRI scans of adolescent brains at 14 and again at 18 to paint a picture of one plausible effect of marijuana.

In the JAMA Psychiatry report published Wednesday, that effect boiled down to this: accelerated thinning of parts of the brain called the right and left prefrontal cortices.

The authors write that their study is the “largest longitudinal neuroimaging study of cannabis use to date” and is particularly pertinent because of what we know about adolescent cannabis use: Most people trying marijuana for the first time (78 percent) are between the ages of 12 and 20.

“What we found was that the more cannabis use, that people were reporting, from 14 to 19, the faster certain cortical areas were thinning,” lead author Matthew Albaugh tells Inverse. Albaugh is a clinical psychologist and assistant professor of psychiatry at the Larner College of Medicine at the University of Vermont.

The findings, he says, “might suggest that cannabis is indeed having some sort of an effect on this neuro-maturation process.”

What you need to know first — Historically, it’s been difficult to study how cannabis affects the brain over time.

Some studies do suggest a link between early cannabis use and altered volume in some parts of the brain, changes in decision-making, and even claim it can moderate a genetic risk for schizophrenia.