The jury’s still out on whether or not marijuana use changes the very structure of the brain, and it seems it’ll continue to be as one study after another contradicts the last one’s findings. In the latest development, a study from Northwestern University finds that teens who smoke marijuana daily not only have worse long-term memory in adulthood but abnormal brain structures as well.

The Northwestern researchers found teens who smoked weed daily for about three years were more likely to experience long-term memory problems in their early 20s, about two years after they stopped smoking altogether. What’s more, they also found these teens’ brains were misshapen, specifically in the hippocampal area, where long-term memory is established and emotions are produced — those with a higher frequency of smoking were more likely to have these abnormalities.

“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,” said senior author Dr. John Csernansky, chair of psychiatry and behavioral sciences at Northwestern’s Feinberg School of Medicine, in a press release.

Previous studies have found similar and conflicting results, however, none have been conclusive. One from 2012, for example, found daily cannabis use lowered IQ and shrunk the brain. Meanwhile, a study published last month in The Journal of Neuroscience found teens and adults who used marijuana on a daily basis experienced “no statistically significant differences… between daily users and nonusers on volume or shape in the regions of interest” — the study looked at the nucleus accumbens, amygdala, hippocampus, and cerebellum. The researchers saw strength in their study because it accounted for alcohol use, which has been shown to change the shape of the brain.  

Speaking about that study to Reuters, Anne Blood, a professor of psychiatry at Harvard Medical School who worked on previous studies with Northwestern University, said the researchers found conflicting results because they didn’t include a healthy, non-substance abusing control group. Other limitations of the study included not accounting for socioeconomic factors or history of marijuana use. So, while the findings may have been positive, they too are mired in uncertainty.

For the current study, the researchers looked at teens who began using marijuana daily between the ages of 16 and 17, and continued to do so for about three years. Two years after the 97 participants gave up on marijuana, the researchers performed brain scans and conducted narrative memory tests, in which participants were asked to listen to several stories then recall content 20 to 30 minutes later. The researchers found overall, participants who used marijuana performed about 18 percent worse on the tests when compared to nonusers.

Still, the researchers admitted their findings weren’t conclusive, as a longer-term study would need to determine if marijuana was responsible for the brain structure differences, and thus memory impairment. “It is possible that the abnormal brain structures reveal a preexisting vulnerability to marijuana abuse,” lead author Matthew Smith said in the release. “But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause.”

Source: Smith M, Lang W, Csernansky J, et al. Hippocampus. 2015.