Smoking marijuana daily for 3 years as a teen is linked with having an abnormally shaped hippocampus and long-term memory problems, according to the results of a new study by researchers at Northwestern Medicine in Chicago, IL.

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The researchers think the hippocampus may be more susceptible to alteration the longer the use of marijuana occurs.

The hippocampus region of the brain is known to be important for regulating emotions and long-term memory. Previous research, such as a 2008 study conducted by researchers at the University of Melbourne, Australia, found an association between structural abnormalities in the hippocampus and long-term, heavy cannabis use.

The Australian researchers found that the hippocampus and amygdala were smaller in cannabis users than in non-users. The cannabis users also demonstrated more “sub-threshold symptoms” of psychotic disorders than the control group who did not use cannabis.

The Northwestern study, published in the journal Hippocampus, used “advanced brain mapping tools” to examine in closer detail the subtle changes in brain regions found in marijuana users.

Previously, the Northwestern team had demonstrated poor short-term and working memory performance in marijuana users, along with abnormalities in the shapes of other brain regions, including the striatum, globus pallidus and thalamus.

“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,” says lead study author Matthew Smith, assistant professor of psychiatry and behavioral sciences at Northwestern’s Feinberg School of Medicine.

The researchers recruited 97 participants for the study, which included people with a marijuana use disorder, schizophrenia patients with no history of substance use disorders, schizophrenia patients with a marijuana use disorder, and matched groups of healthy control subjects.

The participants in the study who used marijuana did not use other drugs and reported using marijuana every day for an average of 3 years beginning at the age of 16 or 17. The participants were in their early twenties at the time of the study and had been marijuana-free for an average of 2 years.

The study shows that the longer the subjects were chronically using marijuana, the more abnormal the shape of their hippocampus. The researchers think the hippocampus may be more susceptible to these changes the longer the use of marijuana occurs.

The authors also believe that the abnormal shape indicates damage to the neurons and axons within the hippocampus.

Participants in the study took a “narrative memory test,” in which they were required to listen to a series of 1-minute stories and then recall as much content as possible 20-30 minutes later.

Overall, the former cannabis users performed 18% worse on memory tests than healthy control subjects. Participants with schizophrenia who had used marijuana in their teenage years were found to perform about 26% more poorly on these tests than peers who had never used marijuana.

However, a longitudinal study is required to show conclusively whether marijuana is responsible for the brain differences and memory impairment found in the study subjects who had used marijuana. Smith explains that the current results are unable to definitively show this, as the study examined just one point in time.

“It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse,” Smith says. “But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause.”