Pot might be toted as a ‘harmless’ drug, but new evidence says otherwise.

By Abigail Thorpe
Updated March 13, 2015
Aaron McCoy

Marijuana is the most commonly used illicit drug in the United States. And its prevalence amongst adolescents and young adults may be particularly alarming in light of a recent study from Northwestern University researchers that suggests extensive marijuana use as a teen can damage long-term memory in adulthood.

“The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems and to sustain our relationships with friends and family,” senior author Dr. John Csernansky said in a statement. The study observed 97 participants, all of whom started using marijuana daily between the ages of 16 and 17 for about three years, and had been marijuana-free for two years or so (they didn’t abuse any other drugs). The participants took a narrative memory test in which they listened to a series of stories for about a minute— 20 to 30 minutes later, they were asked to recall everything they could.

The results showed former users performed 18 percent worse than non-users on long-term memory tests, and also showed a link between an oddly-shaped hippocampus (most likely as a result of marijuana use) and poor long-term memory. The hippocampus is an area of the brain related to long-term memory. The study found that loss in long-term memory was greater the longer the teens used marijuana, and young adults with schizophrenia and a history of marijuana abuse demonstrated an even greater decline.

This study builds on previous work by the research team, which showed the effects of marijuana abuse on short-term memory. “Both our recent studies link the chronic use of marijuana during adolescence to these differences in the shape of brain regions that are critical to memory and that appear to last for at least a few years after people stop using it,” lead study author Matthew Smith said in a statement.

Because the study was relatively small and only focused on one period of time, more research is needed to definitively prove a link—but the evidence is enough to share with your teen in the meantime.