The Government has confirmed a cannabis referendum will be held at 2020 election. Christchurch residents offer their views.

Otago University health researchers warn cannabis risks are being downplayed in talk about changing the law but, in a New Zealand Medical Journal editorial, do support decriminalising recreational use by adults.

A binding referendum on personal cannabis use is being held at the 2020 election. Justice Minister Andrew Little announced the decision shortly before Christmas, at the time saying some details were still to be worked through.

Friday'seditorial in the NZMJ urging caution in how the law is changed was written by Research Associate Professor Joseph Boden, from the university's Department of Psychological Medicine, and by Emeritus Professor David Fergusson, who died in October.

ROSS BELL New Zealand will vote on cannabis law reform at the next election. The picture shows cannabis inside a factory in North America, where sale of recreational marijuana is now legal in 11 states and Canada.

The editorial said an unfortunate feature of the cannabis law debate was that relatively few contributors had talked about either the harms of cannabis, or the potential risks of decriminalisation.

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"Cannabis has multiple harmful effects which are particularly evident for young users, and the extent to which legalisation is beneficial is by no means clear," the editorial said.

JASON FRANSON/AP Customers shop at Numo Cannabis in Edmonton, Alberta, Canada.

"Most contributions (to the debate) imply that cannabis is a relatively harmless drug, and that cannabis law change will only have beneficial consequences.

"We would argue that, on the basis of evidence generated by longitudinal studies based in New Zealand, both assumptions are incorrect."

New Zealand had some of the richest data on the adverse consequences of cannabis use – from the Christchurch Health and Development Study and the Dunedin Multidisciplinary Health and Development Study.

Te Kāea The Medicinal Cannabis Bill has passed its second reading, with a government amnesty giving illicit cannabis growers the opportunity to enter the medicinal cannabis industry.

The Christchurch study involved 1265 people born in 1977 and studied to the age of 35. More than 75 per cent reported using cannabis, with about 15 per cent developing a pattern of heavy use and dependence at some point.

Data showed cannabis use was associated with educational delay, welfare dependence, increased risks of psychotic symptoms, major depression, increased risks of motor vehicle accidents, increased risks of tobacco use, increased risks of other illicit drug use, and respiratory impairment.

The effects were most evident for users aged under 18, and in particular heavier users of cannabis, and could not be explained by social demographic and contextual factors associated with cannabis use, the editorial said.

KAVINDA HERATH A protest to legalise marijuana in front of the Invercargill police station in 2016.

The research clearly established that cannabis use was not harmless, and that it had its greatest impacts on young users who were vulnerable to the neuro-psychological effects.

An analysis of the Christchurch cohort also showed Māori and males reported disproportionately higher rates of arrest and conviction for cannabis-related offences.

"These findings indicated that the laws prohibiting cannabis were being applied in a biased manner," the editorial said.

Some of the advocacy for the legalisation of cannabis had tried to show it was no more harmful, or even less harmful, than alcohol. But the harms of cannabis and alcohol covered quite different domains of functioning.

Also, because cannabis was an illegal drug, harms associated with it may have been underestimated.

In the US, changes in medical and recreational cannabis laws had resulted in mixed effects. Little evidence was found that cannabis use had increased among adolescents. But among adults cannabis use and cannabis use disorders were up.

Any changes to the legal status of cannabis should be made with caution, the editorial said. The authors said their proposal discouraged the use of cannabis, while also avoiding criminalising recreational users of the drug.

They proposed simple possession of cannabis by those aged over 18 should be decriminalised, as would the supply of small amounts to adults. Penalties for supplying cannabis to those under 18 should be increased.

Spending on mental health services for those with cannabis use disorders and cannabis-related conditions should be increased.

If evaluations showed the decriminalisation process reduced harm, cannabis laws could be further liberalised, the editorial said.

Massey University drug researcher Associate Professor Chris Wilkins said part of the problem with the drug reform debate was the extent of polarisation. "There's a need for some kind of balanced evidence-based assessment," he said.

"In that advocacy area I think people have been downplaying cannabis harms to some extent, but also the research is only just starting to catch up."

He agreed that in some ways comparing alcohol and cannabis was difficult, with the risks being different. It was more complicated than that and a deeper level of detail was needed.

There was a reasonably good argument that a stepped approach to decriminalisation could be taken.

"My general approach is that there's a good argument for some sort of reform," Wilkins said. "I wouldn't support the prohibition which we have now, or a fully commercial market like we have for alcohol."

Between those two extremes there were 10 options to consider. One of those was to have a system based on not-for-profit community trusts, with some of the money from cannabis sales being used for local drug treatment and prevention.

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