Policymakers considering legalising cannabis need to take into account evidence about the drug's links to mental health issues, use of other substances and poorer achievement, a researcher says.

The Government has announced it will hold a public referendum for personal use at or before the 2020 general election, as part of its confidence and supply agreement with the Greens.

Associate Professor Joe Boden, of the University of Otago's Christchurch Health and Development Study (CHDS), said his 20-year longitudinal study, following 1000 people in Canterbury, showed more than 80 per cent of middle-aged Kiwis had used cannabis at least once with little long-term harm.

But the study suggested that among young, regular users, the drug could later affect their mental health, use of other illegal drugs and earning potential.


Those who used the drug at least weekly during their teenage years were almost twice as likely as others to experience symptoms of psychosis than infrequent or non-users.

Those who used the drug at least weekly up to age 25 were more than 10 times more likely to use other illegal drugs.

Fewer than 20 per cent of those who started using cannabis before age 15 achieved a tertiary qualification, compared with nearly 30 per cent among those who did not use cannabis before 18.

By age 25, those who used cannabis at least weekly as a teenager were three times more likely to experience long-term unemployment than those who did not use cannabis, or who used cannabis very infrequently.

"The CHDS data, along with data from other New Zealand and international studies, suggests that the harms of cannabis are most pronounced for those who begin using at younger ages, or who use cannabis heavily during adolescence," Boden said.

"Any change to the law concerning cannabis needs to be undertaken in a way to reduce harm in this group, and in particular to provide resources for the treatment of those who develop cannabis dependence."

Boden said the evidence from his research group suggested it was important any law change protected the most vulnerable, and wasn't just a change to alcohol's "open slather free market".

The impact of any law changes needs to be carefully studied, Boden said.


Cabinet had not yet considered the cannabis referendum, but when it did the date would be one of its considerations.

NZ First leader Winston Peters has said he wanted the cannabis referendum to be binding on the Government.

But Justice Minister Andrew Little said neither the cannabis referendum, or that on another on Act Party leader David Seymour's bill to legalise voluntary euthanasia, were likely to be binding.

"We don't typically do binding referenda in this country," Little said.