AS THE world’s drug habit shows, governments are failing in their quest to monitor every London window-box and Andean hillside for banned plants. But even that Sisyphean task looks easy next to the fight against synthetic drugs. No sooner has a drug been blacklisted than chemists adjust their recipe and start churning out a subtly different one. These “legal highs” are sold for the few months it takes the authorities to identify and ban them, and then the cycle begins again. In June the UN reported more than 250 such drugs in circulation.

An unlikely leader in legal highs is New Zealand. Conventional hard drugs are scarce in the country, because traffickers have little interest in serving 4m people far out in the South Pacific. Kiwis therefore make their own synthetic drugs, which they take in greater quantity than virtually anyone else. The government shuts down more crystal-meth labs there than anywhere bar America and Ukraine. But the business has adapted. First it turned to benzylpiperazine, which a third of young New Zealanders have tried. When that was banned in 2008, dealers found plenty of other chemicals to peddle. Today the most popular highs are synthetic cannabinoids, which pack a harder punch than ordinary cannabis.

Sick of trying to keep up with drugmakers, the government is trying a new tack. Last month a law was passed which offers drug designers the chance of getting official approval for their products. If they can persuade a new “Psychoactive Substances Regulatory Authority” that their pills and powders are low risk, they will be licensed to market them, whether or not they get people high. Drugs will have to undergo clinical trials, which the government expects to take around 18 months—much less than for medicines, because the drugs will be tested only for toxicity, not for efficacy. Drugs that are already banned internationally, such as cocaine and cannabis, are ineligible. Only licensed shops will sell the drugs, without advertising and not to children.

The arguments for legalisation—that it protects consumers, shuts out criminals and saves money while raising tax—are familiar to readers of this newspaper. Yet it requires careful regulation to ensure that its outcome is not worse than widely ignored prohibition. New Zealand must now get the details right. The government has yet to define “low risk”. Set the bar too high and the policy will be prohibition by another name; too low and potentially lethal products will be on sale legally. (They are already, in the form of alcohol and tobacco, but consistency is hardly a feature of drug policy.) Nor does anybody know what level of taxation will most effectively deter consumption without encouraging a black market. Similar debates are under way in Uruguay, which is poised to legalise cannabis, and in Colorado and Washington state in America, which voted to do so last year.

Trust the health ministry over the mafia

These tricky questions may look like weaknesses in the policy. In fact, they are its strength. While New Zealand and Uruguay are discussing what level of toxicity or what dosage is acceptable, every other country is leaving the matter to drug dealers, who do not care about quality control and who peddle to children on the same terms as adults. As New Zealand ponders what rate of tax to levy, in the rest of the world the business is tax-free. A hard road lies ahead for New Zealand and its fellow policy innovators. But every dilemma they face is a reminder that, unlike other jurisdictions, through government they are regulating the drugs business, not the gangs.