Heavy marijuana use in the teenage years could damage brain structures vital to memory and reasoning, a new study suggests.

The study found changes in the sub-cortical regions of the brain, which are part of the memory and reasoning circuits, NBC News reports. Young people who had changes in this region of the brain performed more poorly on memory tests than their peers who did not use marijuana. The heavy marijuana users in the study had not used the drug on average for more than two years before the memory testing occurred.

The results appear in the journal Schizophrenia Bulletin.

“We see that adolescents are at a very vulnerable stage neurodevelopmentally,” said lead researcher Matthew Smith of the Northwestern University Feinberg School of Medicine in Chicago. “And if you throw stuff into the brain that’s not supposed to be there, there are long-term implications for their development.”

The study included 10 people with a history of cannabis use disorder, 15 people with a history of cannabis use disorder and schizophrenia, and 28 with schizophrenia but no past regular marijuana use. The study also included 44 healthy people without a history of marijuana use. The participants who had used marijuana had been heavy users in their teen years. Their average age at the time of the study was mid-20s.

The participants’ brains were scanned using MRI. They were then given tests of working memory, such as remembering number sequences. People who had a history of heavy marijuana use, whether or not they had schizophrenia, performed more poorly on the tests. They also showed abnormalities in regions of the brain related to reward and motivation, cognition input and movement and memory.

“We saw poor performance in the marijuana groups…” Smith said. “And the younger somebody started using, the more abnormal they looked.” He noted the study does not prove using marijuana caused the results. He said it is possible the brain differences made heavy users more likely to smoke marijuana in the first place.