Heavy teenage cannabis users can suffer brain damage and long-term memory loss, a new study has found.

Looking at 97 people in their early 20s who used the drug daily for three years in their teens, scientists found they had an odd-shaped hippocampus, which is vital to memory.

They performed 18 per cent worse in long-term memory tests compared to those who had never smoked the drug.

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

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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.

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.

Co-author Dr Matthew Smith, also from Northwestern University, said: ‘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.

‘It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse.

‘But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause.’