POT smokers may want to think twice before lighting up their next joint.

Australian scientists have proved the long-held suspicion that persistent heavy marijuana use damages the brain's memory and learning capacity.

Researchers also showed for the first time the earlier people developed their cannabis habit, the worse the damage.

Scientists from Melbourne's Murdoch Childrens Research Institute (MCRI), Melbourne University and Wollongong University used Magnetic Resonance Imaging (MRI) to scan the brains of 59 people who had been using marijuana for 15 years on average.

The images were compared with scans of 33 healthy people who had never used the drug.

The scans measured changes to the volume, strength and integrity of white matter, the brain's complex wiring system.

Unlike grey matter, the brain's thinking areas which peak at age eight, white matter continues developing over a lifetime.

Senior researcher Dr Marc Seal of MCRI said the scans showed long-term heavy cannabis users had disruptions in their white matter fibres.

There was a reduction in the volume of white matter of more than 80 per cent in the users studied, Dr Seal said.

While the average age participants started using marijuana was 16, some began as young as 10 or 11 and were more seriously affected.

"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," Dr Seal told AAP.

Cannabis interferes with naturally occurring cannabinoid receptors in the brain.

"If you're a teenager and you've got all these natural cannabinoids in your white matter, it's not good to be introducing a lot of external cannabinoids in your system, because it stops the white matter maturing," Dr Seal said.

The significant differences in long-term heavy cannabis users' white matter was linked to poor memory and learning.

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

"These differences are linked to memory impairment and concentration.

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

The results could not be explained by other recreational drug use and alcohol.

Dr Seal said the participants would be followed up in the next two years to track any further changes.

The results added to previous evidence showing the hippocampus, the area of the brain involved in memory, shrunk in heavy users.

Previous studies investigating white matter among cannabis users had substantially smaller numbers of participants.