The Prime Minister is unmoved by a poll for the Drug Foundation showing most New Zealanders now support legalising, or at least decriminalising, cannabis. His reluctance is understandable. The last time the Government went down this road, with synthetic cannabis, it got its fingers burned.

It is easy to agree, as 64 per cent of those polled did, that possession for personal use should not be a crime. It is almost as easy to agree with the 52 per cent who would decriminalise its cultivation for personal use. But if the purpose is to save police time, it would probably do the reverse. How would police know a person found in possession of cannabis, or cultivating it, had no more than an amount permitted for personal use? And if cannabis was merely decriminalised, remaining illegal, what are police supposed to do about it? Their job is easier when crime is clearly defined.

But ease of law enforcement is not the decisive consideration. The law should regulate drugs to the extent necessary to minimise the harm they can do. The case for relaxing the law against cannabis has been helped of late by cancer patients who find it an effective pain reliever. Unsurprisingly, more than 80 per cent of those polled agree cannabis should be permitted for pain relief but, again, changing the law is the easy part. How are people in pain going to be supplied? Must they all grow their own?

The Drug Foundation's poll finds much less support (30 per cent) for legalising or decriminalising commercial sales of cannabis for any purpose and, oddly, even less support (20 per cent) for allowing small amounts to be grown and sold or given away to friends. Most of us, it seems, want any liberalisation of cannabis to ensure it strictly remains a personal and private activity for those who want it, not a substance for social sharing or commercial promotion. But if it is going to be available for medical use it will need some sort of organised production and distribution.

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When advocates such as the Drug Foundation urge governments to treat cannabis as a matter for health regulation rather than the criminal law, they envisage licensing its manufacture, testing and controlling the safety and strength of approved products, restricting the number and location of outlets and regulating its promotion and terms of sale, much as is done for alcohol and tobacco.

That is exactly the route suggested to the Government when synthesised equivalents appeared on the market some years ago. The Cabinet agreed to set up a licensing regime and some products were given interim approval while health officials set about working out tests for safety. Councils, meanwhile, were given power to decide where the drugs could be sold. The experiment was soon a political disaster. The few approved outlets became unsavoury places and the permitted drugs were anything but safe, according to parents of users who now blamed the Government for their availability.

Their approval was withdrawn. A testing regime for product safety has yet to appear. Legalising drugs is more difficult than it sounds.