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Marijuana Use May Lead to Poor Long-Term Memory

Mar 12, 2015 05:05 PM EDT

Marijuana use as a teenager may lead to poor long-term memory in one's adult years, according to a new study.

Our ability to recall autobiographical or life events after a long period of time (episodic memory), is due to the brain region known as the hippocampus.

But researchers from Northwestern University are among the first to not only find that the hippocampus is abnormally shaped in heavy marijuana users - those who smoke it daily for about three years - but that it is also directly related to poor long-term memory performance. This is hardly the first time scientists have suggested such a link, but it is the first time they've found solid evidence of their suspicions.

"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 from Northwestern University said in a press release.

According to the results, published in the journal Hippocampus, young adults who abused cannabis as teens performed about 18 percent worse on long-term memory tests than young adults who never used cannabis. What's more, the longer they used the drug, the greater the differences were in hippocampus shape.

The abnormal shape likely reflects damage to the hippocampus and could include the structure's neurons, axons or their supportive environments.

During the study, participants (age 16-17) took a narrative memory test in which they listened to a series of stories for about one minute. They were then asked to recall as much content as possible 20 to 30 minutes later. Nearly 100 subjects participated, all of whom had not smoked marijuana for about two years prior to the study. But at the beginning of the study, the research team asked the teens to start using marijuana daily for the next three years.

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

Marijuana is currently the most commonly used illegal drug in the United States, particularly among young adults. And as the demand for cannabis grows - four states have legalized it for recreational use, while 23 states (and Washington DC) have legalized it for medical use - so may issues with long-term memory.

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