Heavy marijuana use damages brain's memory and learning capacity, say researchers.
They 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 showed long-term heavy cannabis users had disruptions in their white matter fibres, said senior researcher Dr Marc Seal of MCRI.
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," News.com.au quoted Dr Seal as telling a foreign news agency.
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.