Washington: Teens who are heavy marijuana users - smoking it daily for about three years - may have an abnormally shaped hippocampus and poor long-term memory, a new study has found.
Researchers from Northwestern University found that young adults who abused cannabis as teens performed about 18 per cent worse on long-term memory tests than young adults who never abused cannabis.
The brain abnormalities and memory problems were observed during the individuals' early twenties, two years after they stopped smoking marijuana.
"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 author Dr John Csernansky, the Lizzie Gilman professor and chair of psychiatry and behavioural sciences at Northwestern University Feinberg School of Medicine and Northwestern Memorial Hospital.
The study is among the first to find that the hippocampus is shaped differently in heavy marijuana smokers and the different looking shape is directly related to poor long-term memory performance.
Previous studies of cannabis users have shown either the oddly shaped hippocampus or poor long-term memory but none have linked them.
Previous research by the same Northwestern team showed poor short-term and working memory performance and abnormal shapes of brain structures in the sub-cortex 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," said lead study author Matthew Smith, assistant professor of psychiatry and behavioral sciences at the Feinberg School of Medicine.
The longer the individuals were chronically using marijuana, the more abnormal the shape of their hippocampus, the study found.
The findings suggest that these regions related to memory may be more susceptible to the effects of the drug the longer the abuse occurs.
The abnormal shape likely reflects damage to the hippocampus and could include the structure's neurons, axons or their supportive environments.
Because the study results examined one point in time, a longitudinal study is needed to definitively show if marijuana is responsible for the observed differences in the brain and memory impairment, Smith said.
"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," he said.