Researchers have found that teens who smoked marijuana daily for several years have an abnormally shaped hippocampus and do poorly on tasks involving long-term memory.

A study published in the journal Hippocampus reveals that abnormalities in the brain and problems with memory manifest in the early 20s for pot-smoking teens roughly two years after quitting. When tested on their long-term memory, those had used marijuana heavily as teens performed around 18 percent lower than those who didn't abuse marijuana.

Abundant marijuana use appears to affect the memory processes used for social interaction and solving day-to-day problems. Other studies have found either poor long-term memory performance or irregular hippocampus shape in marijuana users, but this is the first time the two have been linked.

The study found that the subjects who had used marijuana for the longest also had the most abnormality in the shape of the hippocampus. This suggests that memory-related parts of the brain are the first to be affected by cannabis, and become more susceptible with persistent heavy use. 

The participants in the study started using marijuana when they were 16 or 17, continuing with daily use for three years. The 97 individuals included healthy control subjects, subjects with marijuana use disorder, schizophrenia subjects with marijuana use disorder, and schizophrenia subjects with no history of substance use disorder. None of the subjects abused other drugs, and were marijuana free for around two years at the time of the study.

For the memory test, the subjects listened to a number of one-minute stories. They were then asked to retell the stories 20 to 30 minutes later, reproducing as much information as possible. The test was designed to assess the ability to recall, store and encode details from the stories.

Matthew Smith, the lead author for the study, said that a longitudinal study would be needed to confirm that marijuana is indeed the cause of hippocampus abnormalities and memory loss. 

"It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse," 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 legal for recreational use in four states, and 23 states and Washington, D.C. allow its use for medical purposes.

The study received funding support from the National Institute of Health's National Institute of Mental Health. Other authors include: John Csernansky, Hans Breiter, Lei Wang, Kathryn Alpert, Andrea Roberts, Jodi Gilman, James Reilly and Derin Cobia.

Photo: Torben Hansen | Flickr

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