WASHINGTON -Teenagers who are heavy marijuana users have ‘abnormal brain structures and poor memory.’ A new US study found young people who had smoked cannabis daily performed worse on memory tests, despite not having used the drug for over two years.

Changes in their brains related to the regions responsible for the ability to retain and process information. These regions appeared to shrink and collapse inward, possibly reflecting a decrease in neurons - similar to schizophrenia-related brain abnormalities. The research at Northwestern University in Illinois tested 44 non-using healthy people in their 20s and compared them with 10 young people with a history of cannabis use.

The group also tested 15 young people with a history of cannabis use and schizophrenia, and 28 with schizophrenia and no past regular use of marijuana. When the team made MRI scans of three sub-cortical brain regions, heavy users showed abnormalities, whether or not they had schizophrenia. This is the first study to target key brain regions in the deep subcortical grey matter of chronic marijuana users.

The younger the individuals were when they started chronically using marijuana, the more abnormally their brain regions were shaped, the researchers said. The scientists, however, added the study leaves key unanswered questions. Northwestern University Professor Matthew Smith said the differences in brain geography could have existed before the young people used cannabis. It is possible that their brain differences made them more likely to smoke pot in the first place. ‘With the movement to decriminalise marijuana, we need more research to understand its effect on the brain,’ he said.

But chronic use of marijuana may also contribute to changes in brain structure that are associated with having schizophrenia, the Northwestern researchers said. Of the 15 marijuana smokers who had schizophrenia in the study, 90 per cent started heavily using the drug before they developed the mental disorder. Marijuana abuse has been linked to developing schizophrenia in prior research. ‘The abuse of popular street drugs, such as marijuana, may have dangerous implications for young people who are developing or have developed mental disorders,’ said co-senior study author John Csernansky.

‘This paper is among the first to reveal that the use of marijuana may contribute to the changes in brain structure that have been associated with having schizophrenia.’ ‘If someone has a family history of schizophrenia, they are increasing their risk of developing schizophrenia if they abuse marijuana,’ added Professor Smith.