Scientists from the Murdoch Children's Research Institute of the Melbourne University had established a link between the heavy use of marijuana and damage to brain memory and learning capacity.

The bases of their findings are comparative Magnetic Resonance Imaging (MRI) scans of brains of 59 people who used marijuana for an average of 15 years and 33 people who did not.

The worst damage was experience by pot smokers who started the vice early. The MRI scans measured changes to volume, strength and integrity of white matter which is the brain's complex wiring system. White matter develops continuously over the lifetime of a person unlike grey matter, the thinking areas, which reach a peak at age eight.

The scans showed that heavy marijuana users had disruptions in the fibres of their white mater and reduction in volume by over 80 per cent. These effects of pot use were worse for those who started the habit as early as 10 or 11 years old, said Dr Marc Seal, senior researcher of the institute.

"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," Mr Seal told AAP.

He said the effects such as poor memory and learning are quite significant, but he was unsure if these are irreversible.