Heavy marijuana use in adolescence may lead to abnormal changes in the parts of the brain related to memory, a new study suggests.

US scientists looked at almost 100 people, some of whom had used marijuana and some who had not. Among those who used it, the drug was taken on a daily basis for around three years from the age of 16 or 17. No other drugs were taken and at the time of the study, they had not used marijuana in two years.

The scientists found that those who were heavy users of the drug in their teens had abnormal changes in the parts of the brain that are related to working memory. This is the part of the memory that deals with remembering and processing information in the moment, and if required, transferring it to long-term memory.

These parts of the brain actually appeared to get smaller and collapse inward.

Not surprisingly, heavy users of the drug performed badly in memory tests.

These problems were picked up when the participants were in their early 20s, two years after they had stopped using the drug. This, the scientists said, suggests that early chronic use of marijuana could have long-term effects.

The study found that the younger a person was when they started heavily using marijuana, the more abnormally shaped these parts of the brain were found to be.

"The study links the chronic use of marijuana to these concerning brain abnormalities that appear to last for at least a few years after people stop using it," commented the study's lead author, Dr Matthew Smith, of Northwestern University in Chicago.

He insisted that with increasing calls to decriminalise marijuana, ‘we need more research to understand its effect on the brain'.

The study also found that heavy marijuana use may lead to changes in the brain that are associated with schizophrenia. Of the people who had schizophrenia in this study, 90% started using marijuana heavily before they developed the mental health disorder.

A link between marijuana and schizophrenia has been suggested in previous studies.

"The abuse of popular street drugs, such as marijuana, may have dangerous implications for young people who are developing or have developed mental disorders. 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," said co-senior study author, Dr John Csernansky.

Details of these findings are published in the journal, Schizophrenia Bulletin.