A new study published last week in the journal Hippocampus finds that young adults who smoked cannabis frequently as teens performed 18 percent worse on memory tests than young adults who did not smoke. Researchers also found the former smokers’ hippocampuses were abnormally shaped.

The patients studied were in their early 20s and all smoked cannabis everyday for 2-3 years between the ages of 16 and 17. At the time of the study, they had been off the drug for over 2 years. All patients only used marijuana and did not abuse any other type of drugs.

The memory tests involved listening to a few stories for one minute. Then 20 to 30 minutes later, they were required to recall as much content as possible. Results showed the participants who were chronic smokers scored lower than non smokers.

Brain mapping tools allowed the researchers to also study the shape of all the participant’s brains. Those who smoked marijuana had abnormally shaped hippocampuses, which is the part of the brain that controls memory. Sometimes the abnormality was small, but it was still there.

The abnormality in the hippocampus could be linked to the poor memory in the tests. Research has been done on memory and the hippocampus separately, but this is one of the first studies that link the two.

More studies will need to be done to further confirm this link.