The 2020 referendum to legalise cannabis looks likely to propose a tightly regulated framework, including strict rules on supply and possession, an age limit of at least 18, and a non-profit model where money from sales may be funnelled into health services.

And while it is widely accepted that legalising personal use would not eliminate harm or kill off a black market, a political consensus appears to be emerging that the status quo is broken, but a profit-driven legal market would be just as bad.

Justice Minister Andrew Little said the Government was still working on the referendum question, but he personally opposed a framework similar to alcohol if the public voted for legalisation in 2020.

"My general view would be, if there is an appetite for liberalisation in whatever form, to start with maximum regulation and control," Little told the Weekend Herald.

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"That's the way you mitigate the risks, and then future generations can review what's happening and whether further relaxation is needed."

This sentiment was echoed by NZ First justice spokesman Darroch Ball, who said the level of cannabis-related harm in a black market dominated by gangs showed that the current system was not working.

"But we all understand that marijuana is a drug, and it's not all positives.

"It's just common sense to start at a more conservative, regulated market. Once you have no regulation, the horse has bolted and there's no coming back."

He added that the NZ First caucus was yet to make any decisions about the referendum question.

Green Party spokeswoman for drug law reform Chloe Swarbrick said a black market and a legal free market both "preyed on vulnerable people".

She pointed to a report from the Global Commission on Drug Policy, which said both models were "extremes" that pursued profit without having to deal with social harms, while "an optimal level of government regulation can minimise overall harm and maximise benefits".

"Regulation means quality control. We have no control over it right now," Swarbrick said.

This week National Party deputy leader Paula Bennett was appointed its spokeswoman for drug reform, and she warned about the perils of a profit-driven model.

"They're turning marijuana products into lollies and ice cream so that it attracts younger people and gets them hooked. It then becomes a business ... We're hearing about a major beer brewer also looking at a cannabis-based product. Those things frighten me."

Little was reluctant to detail what "maximum control" might look like, but suggested that the alcohol industry was the horse that bolted and "we're not going to repeat the mistakes of the past".

There are several issues around regulation to consider, including:

• a purchase age (at least 18 in all jurisdictions that have legalised)
• who supplies the market
• whether home-grown cannabis should be legal
• a limit on the amount a person can have on them
• the location of outlets, including proximity to schools
• the level of advertising or promotion
• pricing and taxes, and whether any money from sales should be spent on health services
• the allowable level of THC for driving
• whether cannabis-related convictions should be pardoned or expunged, or disqualify a person from working a legal market
• the level of penalties for breaking the rules

Drug Foundation executive director Ross Bell said an example of "maximum control" would be to select the strictest measures from jurisdictions that have legalised cannabis.

That could translate to a government monopoly on supply and a national register for all users (Uruguay), a ban on home cultivation and public consumption (Washington state), a purchase age of 21 (many US states, in line with the respective drinking age in those states), a weekly purchase limit of 10g (Uruguay), a limit on the level of THC in the blood to be legally allowed to drive (Canada and Colorado), and a blanket ban on advertising.

But Bell said any controls had to consider whether they would create a legal vacuum that the black market could fill; an age limit of 25 and banning highly potent products, for example, could lead to criminals meeting that consumer demand.

How rules can be enforced was also a factor.

"How do you limit youth access when you allow people to grow it in their own home, and how do you police home-grown when people do that now already?"

Bell also supported a regulated, non-profit model with smaller community providers rather than "coca-cola type" suppliers.

"We have to learn the lessons from tobacco and alcohol policy. We're now spending the last 20 years trying to tighten those markets.

"The best way to avoid that is to start strict and to loosen, if necessary, over time."

A poll conducted by Horizon and released at the end of last year found some public support for regulation; 63 per cent wanted licenced operators; 75 per cent wanted a purchasing age of either 18 or 21; 40 per cent wanted an excise tax, and 68 per cent said tax revenue should go towards health services.

How to prevent young people from accessing illegal cannabis was one of the major concerns.

Drug researcher and Massey University Associate Professor Chris Wilkins said teenagers were particularly at risk.

"There is a higher risk of dependency, educational underachievement, unemployment, and adverse effects on brain development and IQ, particularly around 14 or younger."

Regulation was one way to mitigate that risk because it would control a market that would otherwise see increased cannabis availability, lower prices, and "corporations pushing to promote cannabis to young people".

Wilkins supported a non-profit model similar to gambling slot machines, where 40 per cent of all sales are returned to the community "to support things like sports, arts, culture, but also drug treatment and drug prevention".

Studies into the effects of legalisation on cannabis use have shown mixed results.

The US National Survey on Drug Use and Health showed that marijuana usage in the past 30 days increased from 10 to 16 per cent among those aged 12 and over from 2008/09 to 2016/17 in Colorado, where recreational cannabis was legalised in 2013.

That increase came mostly from those aged over 18 (10 to 17 per cent), while usage among those aged 12 to 17 dropped (10 to 9 per cent).

A McMaster Health Forum paper, released in 2017, looked at reviews and studies of 35 jurisdictions that had legalised or decriminalised cannabis.

It found an increase in the use of cannabis, but "among those studies that found increased use, the findings are not conclusive".

"The evidence of increased cannabis use is less clear when the long-term trend is considered," the paper said, noting that some studies showed an uptake in use by teenagers that dissipated after five years.

The paper also noted studies that found increased reports of cannabis-induced visits
to the emergency room, and a greater number of telephone calls to poison control centres following children's accidental ingestion of cannabis.

Little said he would be looking closely at the effects of legalisation overseas as the Government considers the referendum question.

One option he is considering is to prepare a bill setting out a regulatory framework, and then framing the question around support for the bill.

That would allow for a select committee process and public consultation, before Parliament decided on the level of regulation to put to the public vote. It would only become law if a majority voted for legalisation.

This option is supported by the Drug Foundation, legal experts Andrew Geddis and Graeme Edgeler, and the Green Party.

Swarbrick said having a bill in place meant the will of the people could be implemented quickly.

"It wouldn't be up to the next iteration of Parliament to discern what a bill should look like and allow them to kick the can further down the road."

One contentious issue is whether people with past cannabis convictions should be able to supply a legal cannabis market.

That is likely to be opposed by National, which has already proposed preventing anyone associated with a gang or with a drug conviction in the last seven years from working in the medicinal cannabis market.

Swarbrick, Bell and Wilkins all see it as an issue of equity for people who they say have been disproportionately punished by prohibition.

"It's crazy to establish a legal sector and then ban everyone in the black market from participating," Wilkins said.

"They will just continue to grow cannabis illegally. If we don't let them transition into a legal economy, that's not very effective and it's not very fair."