In an April 20, 2005 file photo, a University of Colorado freshman, who did not want to be identified, joins a crowd smoking marijuana during a '420' gathering. Richard M Hackett/AP

Teens who smoked marijuana on a daily basis had abnormal brain structure changes, damaging their memory, after long-term use, a Northwestern University Medicine study found.

The younger the individuals were when they started regularly using marijuana, the more abnormalities were found in their brains – suggesting the brain may be more susceptible to the effects of a drug if the habit starts at an early age.

The individuals observed in the study began using marijuana at ages 16 and 17 and smoked every day for about three years.

It is the first study to target regions deep in the brain related to motivation and memory with MRI technology, and to correlate abnormalities in those regions with an impaired working memory.

Working memory is the ability to remember and process information in the moment and, if needed, transfer it to long-term memory.

After chronic, prolonged cannabis use, the structures related to working memory in the teens' brains appeared to shrink and cave in. Some scientists believe changes in brain structure can change the way the brain functions.

The brain abnormalities and memory problems were observed after the teens had stopped smoking marijuana for two years – indicating long-term effects of regular use.

“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,” said lead study author Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine.

“With the movement to decriminalize marijuana, we need more research to understand its effect on the brain.”