A new study has found that teenagers who heavily smoke cannabis has a misshapen part of their brain which could affect memory.

Teenagers who smoke a lot of cannabis may suffer brain damage and experience long-term memory loss as young adults, a study has suggested.

People in their early 20s who had taken the drug daily for about three years in their teens were found to have an abnormally shaped hippocampus, a brain region vital to memory.

They also performed about 18 per cent worse in long-term memory tests than individuals who had never smoked cannabis.

Professor John Csernansky, from Northwestern University in the US, who co-led the research, said: "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."

The study, published in the journal Hippocampus, is one of the first to suggest that the brain region is shaped differently in heavy cannabis users.

The altered shape is directly related to memory impairment.

Among the study participants, the longer individuals had been exposed to cannabis the more abnormal the hippocampus in their brains appeared.

The scientists used new computer software to fine-map magnetic resonance imaging (MRI) brain scans.

Participants also took a narrative memory test in which they listened to a series of stories for about one minute before recalling as much of their content as possible 20 to 30 minutes later.

A total of 97 individuals took part in the study.

They included people who started smoking cannabis daily between the ages of 16 and 17 and continued for about three years.

At the time of the study, they had been cannabis-free for about two years.

Previous research by the same team has linked poor short-term and working memory performance to abnormal shapes of three other brain regions, the striatum, globus pallidus and thalamus.