Brain abnormalities that make teenagers more likely to smoke cannabis have been identified for the first time by Australian scientists.
Their study of more than 100 Melbourne teens also confirmed that cannabis harms the brain, adding weight to a raft of previous research on damage caused by long-term use of the drug.
Researchers from Monash and Melbourne universities took high-tech images of the brains of 155 primary school students when they were 12.
Four years later when they reached their milestone 16th birthday, the students were asked whether they had used cannabis.
Of 121 who responded, 28 admitted to using the drug.
When the researchers checked the scans taken when those students were 12, they found a part of the frontal lobe area in their brains was smaller than those in teens who steered clear of cannabis.
Lead researcher Professor Lubman, of Monash University, said the students with abnormalities in the orbitofrontal cortex - the brain region involved in memory, reward and decision making - were more prone to using cannabis.
"What we found is that only the orbitofrontal cortex predicted later cannabis use, suggesting that this particular part of the frontal lobe increases an adolescent's vulnerability to cannabis use," he said.
"However we also found no differences in brain volume in other parts of the brain that we have shown to be abnormal in long-term heavy cannabis users, confirming for the first time, that cannabis use is neurotoxic to these brain areas in humans."
Previous studies have shown that long-term heavy cannabis use harms the brain.
Adolescent users have been found to have difficulties with paying attention, solving problems and with their memory.
However the study is the first to examine whether existing brain abnormalities have a role in whether teens start using cannabis.
It was published online by the journal Biological Psychiatry.