Teen cannabis use is far from harmless, according to a landmark study that found even those who smoked it only occasionally were at higher risk of school dropout, suicidal behaviour and other drug use.
In one of the biggest studies of its kind into Australia's most widely used illicit drug, researchers, including Curtin University's National Drug Research Institute, looked at cannabis use in 3765 teens aged under 17 and tracked them to the age of 25.
Writing in The Lancet, researchers said teenagers who had cannabis daily were seven times more likely to try to take their own life, 18 times more likely to become dependent on cannabis and eight times more likely to use other illicit drugs.
Even accounting for factors such as wealth and family issues, teen users were 60 per cent more likely to drop out of high school.
The researchers said the risk of poor educational and health outcomes increased with more frequent cannabis use but findings showed there was no safe level.
Even those who smoked it less than once a month had double the rate of cannabis dependence by age 25 and were 62 per cent more likely to attempt suicide.
Lead researcher Edmund Sil-ins, from the University of NSW, said yesterday the results were strong evidence that teenage cannabis use was a direct cause of problems in young adult life, including mental health issues.
"The findings are timely, given there are some moves to decriminalise or legalise cannabis, which might make it more accessible," Dr Silins said.
The more cannabis was used in adolescence the worse off the user but there was still an effect from once-a-month a use.
Co-researcher David Fergusson, of New Zealand's University of Otago, said the risks from teen cannabis use were specific to that drug and different from alcohol.
He said though drinking was linked to problem behaviour such as aggression, the fallout from using cannabis included suicidal tendencies and low educational achievement.
An estimated one in 25 Australian teenagers aged 14 to 19 use cannabis weekly.
If you or someone you know is thinking of suicide, phone Lifeline on 13 11 14