Teenagers who are heavy marijuana users have poor long-term memory, says a new study.
The longer the individuals were chronically using marijuana, the more abnormal the shape of their hippocampus, the study reported.
The abnormal shape likely reflects damage to the hippocampus and could include the structure's neurons, axons or their supportive environments.
The hippocampus is important to long-term memory (also known as episodic memory), which is the ability to remember autobiographical or life events.
"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 study author John Csernansky from Northwestern University, Chicago.
The brain abnormalities and memory problems were observed during the individuals' early 20s, two years after they stopped smoking marijuana.
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Young adults who abused cannabis as teens performed about 18 percent worse on long-term memory tests than young adults who never abused cannabis.
"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," said lead study author Matthew Smith.
The study appeared in the journal Hippocampus.