Scientists have confirmed the long-held suspicion that frequent heavy marijuana use damages the brain's memory and learning capacity.  

Australian researchers have showed for the first time that the earlier people start their marijuana habit, the worse the brain damage.

"Our results suggest that long-term cannabis use is hazardous to white matter in the developing brain.  This was especially true for those who had started in adolescence, as we know the brain is still developing during this time," Lead researcher Dr. Marc Seal, from Melbourne’s Murdoch Children's Research Institute said in a university release.

Scientists from MCRI, Melbourne University and Wollongong University compared MRI scans of the brain for 59 people who had been using marijuana for an average of 15 years to 33 healthy people who had never used the drug.

After measuring changes to the volume, strength and integrity of white matter in the brains of all participants, researchers found that long-term heavy cannabis users had disruptions in their white matter fibers.

The brain's white matter is responsible for information passed between different areas of grey matter within the nervous system, and unlike grey matter, which are the brain's thinking areas that peaks at age eight, white matter continues to develop as people age.

Seal and his team found that there was more than 80 percent reduction of white matter in the brains of users.

Additionally, researchers found that the average age of participants in the study started using cannabis when they were 16 years old, participants who started using the drug at a younger age like 10 or 11 had even more severe brain damage.

"This is the first study to demonstrate the age at which regular cannabis use begins is a key factor in determining the severity of the brain damage," Seal said, according to AAP.

He explained that marijuana interferes with naturally occurring cannabinoid receptors in the brain and by introducing external cannabinoids into a person's system it stops their white matter from maturing.

Researchers linked the significant changes in the white matter in the brain's hippocampus and commissural fibers, suggesting that long-term marijuana use may lead to memory impairment and deficits in learning and concentration ability.

"These people can have trouble learning new things and they are going to have trouble remembering things," Seal said.

"We don't know if the changes are irreversible but we do know that these changes are quite significant," he added.

Researchers said that the findings could not be explained by recreational drug and alcohol use. Researchers will monitor participants for the next two years to detect any further changes.

The latest findings add to results from previous smaller studies that showed that the brain's memory center, the hippocampus, shrunk in heavy marijuana users.