Otago researchers today told Prime Minister John Key that older teenagers aged between 18 and 21 years should get the same light-handed treatment many teenagers under 16 years get for possession of cannabis.
"The New Zealand youth justice system has evolved a system in which the majority of young people coming to attention are dealt with by diversion rather than prosecution," said Otago University researchers David Fergusson and Joseph Boden.
"There is a clear case for extending these provisions to older adolescents".
While there was increasing evidence of the damage the drug does to some users, a lot of teenagers using it a little were unlikely to be harmed.
But the scientists also noted that teens under 18 were most vulnerable to the effect of cannabis, and sentencing of drug pedlars convicted of supplying cannabis should provide for more severe penalties for providing it to teens.
Improving the Transition - a 300-page report advocating increased use of scientific evidence to underpin government policies to reduce the social and psychological problems of the nation's teenagers - was pulled together by Mr Key's chief science adviser, Sir Peter Gluckman, and a panel of hand-picked experts.
"It is evident that large numbers of young people use cannabis only occasionally and for this group it is unlikely that cannabis has substantial harm effects," said Professor Fergusson and Mr Boden.
"Much of the problematic use of cannabis is likely to be confined to the 10 percent -15 percent of the adolescent population who use cannabis in a heavy and abusive way".
They warned of "complex issues" surrounding the legal regulation of a drug that was widely used "but whose harmful effects are largely confined to a minority of heavy and regular users".
By the age of 21 nearly 80 percent of young people had used cannabis at least once and 10 percent had become dependent, more commonly males and Maori youth.
Risks of cannabis use, abuse and dependence were higher among young Maori.
Regular, heavy or abusive use of cannabis was associated with increased risks of symptoms of psychosis and psychotic symptoms.
And "there is growing evidence to suggest that the use of cannabis may increase the risks of using other illicit drugs", the researchers said.
But there was limited evidence for the effectiveness of drug education in reducing risks to teenagers.
Future policy developments should consider a re-assessment of existing cannabis laws, evaluation of the effectiveness of drug education programmes in cutting risks of illicit drug use, abuse and dependence; and the development of best-practice guidelines for the treatment of young people having cannabis-related problems.
Cannabis is classified as a class C drug with possession attracting a fine of up to $500 or 3 months imprisonment and supplying a prison sentence of up to 14 years, but the researchers said the laws were inefficient and ineffective.
Their Christchurch Health and Development Study had shown that males, Maori and people with a previous criminal record were more likely to be convicted.
There were also problems with proposals to legalise cannabis, including an international treaty which would make it "virtually impossible".
A middle path might be reforms seen in Western Australia, which had a fine of $A100 - $A200 for possession of smoking equipment, up to 30g of marijuana, or two non-hydroponic cannabis plants.
An alternative to a court appearance was to attend a cannabis education session.