When journalists asked Bill English last week if he had any plans to legalise cannabis, the answer would have surprised no one. The Prime Minister replied: "In New Zealand we have always taken the view that some of these drugs cause so much harm that they should be illegal."

English's stance has frustrated many drug reform advocates, who believe the worldwide war on drugs has not only failed but is now in full retreat. They argue that the multibillion-dollar global drug trade is growing stronger than ever in the hands of organised crime and users are criminalised, which makes it harder for them to get help. Drug use, they say, should be a health issue, not a criminal issue.

The message is starting to have an impact. Five American states have legalised cannabis for recreational use, including Nevada last week. Next year Canada will follow suit. Many countries, including Germany and Ireland, are legalising medical marijuana. The extreme example is Portugal, which decriminalised the possession of all drugs for personal use in 2001 but insisted on a medical assessment for every user. It has since had a fall in HIV infections and drug-related deaths.

Associate health minister Peter Dunne has been emboldened by this worldwide trend to suggest decriminalising cannabis here. Dunne would follow the Portuguese model but also allow cannabis sellers, along with other drug manufacturers, to submit their products for testing before sale.

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Dunne was shut down instantly by English but has also failed to win much wider political support, presumably because party leaders know a minefield when they see one. New Zealand has already flirted with legalising synthetic cannabis in 2013, with disastrous results. The public outcry put an end to this country's short-lived plans for a legal market in soft drugs.

The experience should make us wary about overseas claims for legalisation. For instance teen cannabis use in Colorado may have fallen but that flies in the face of our legal highs debacle and human nature in general. If an illegal substance is made legal, usually more people will want to try it, as they no longer fear the social stigma or a criminal conviction.

The theory that legalisation will drive out drug-related crime may also be overstated, as illegal suppliers who pay no tax and have lower overheads are likely to undercut licensed drug traders and target price-sensitive young buyers.

Even Maori Party leader Marama Fox, who supports decriminalisation, is worried about increasing the availability of a drug that has devastated many Maori families. "It is not harmless," she said at a drug symposium last week. "And I think if we ignore that part of the conversation then we are not being upfront with ourselves."

Dunne deserves credit for trying to fix a broken system. But New Zealand's best course of action for now would be to wait and see if the Canadian-American experiment works or not.