Teenage hemp smokers could be damaging brain structures critical to memory and reasoning, according to new research that found changes in the brains of heavy users.

Research released Monday in the journal Schizophrenia Bulletin showed the brains of young heavy marijuana users were altered in so-called sub-cortical regions — primitive structures that are part of the memory and reasoning circuits. And young people with such alterations performed worse on memory tests than non-using controls, despite the fact that the heavy users had not indulged for more than two years, on average, before the testing.

“We see that adolescents are at a very vulnerable stage neurodevelopmentally,” said Matthew Smith, who led the research team at 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.”

Adults who smoked pot regularly as teens were shown to have “neuropsychological decline” and “more cognitive problems” than non-users in a study last year in the Proceedings of the National Academy of Sciences (PNAS). This was true even if users stopped using long before the study.

“It’s important to look at development at young ages,” Dougherty said. “People don’t recognise there is such a huge potential to affect the long-term brain.”