New research suggests that teens who are heavy marijuana users -- smoking it daily for about three years -- are more likely to have memory problems later in life.

Researchers at Northwestern University found that heavy marijuana users had an abnormally shaped hippocampus, an area of the brain that gives people the ability to remember autobiographical or life events, and performed poorly on long-term memory tasks.

"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," Dr. John Csernansky, senior author of the study, said in a statement.

The brain abnormalities and memory problems were observed during the individuals' early twenties, two years after they stopped smoking marijuana. Young adults who abused cannabis as teens performed about 18 percent worse on long-term memory tests than young adults who never abused cannabis.

The study is among the first to say 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.

The study found that the longer the individuals were chronically using marijuana, the more abnormal the shape of their hippocampus. The study suggests that these regions related to memory may be more susceptible to the effects of the drug the longer the abuse occurs.

The findings are detailed in the journal Hippocampus.