UNTIL recently it seemed that nothing would disturb the international consensus that the best way to deal with narcotic and psychotropic drugs is to ban them. Codified in a United Nations convention, this policy has proved impervious to decades of failure. Drug consumption has not, in most parts of the world, fallen. Prohibition inflicts appalling damage, through the spread of organised crime, the needless deaths of addicts exposed to adulterated drugs and the mass incarceration of young men. Now a whiff of change is in the air (see article). Officials in two American states, Colorado and Washington, are pondering how to implement their voters’ decisions in referendums last November to legalise marijuana (cannabis). A dozen countries in Europe and the Americas have deemed the possession of some drugs no longer to be a criminal offence. A few Latin American presidents want a rethink of the “war” on the supply and trafficking of drugs. Several forces are bringing change. First, public attitudes are starting to shift. Americans have seen that the widespread availability of marijuana for ostensibly medical use has not led to mass addiction. Polls show that around half now support full legalisation. In Britain, a poll this week found a similar proportion in favour of decriminalising cannabis possession. Latin America is also tiring of trying to suppress production. That is not surprising: in several countries, the death toll associated with efforts to combat the drug business has risen to the level of a conventional war. Mexicans complain that the notion of “shared responsibility” proclaimed by international bureaucrats means that their people get killed whereas the United States, with its soft gun laws, arms the traffickers, launders their money and consumes their product.

Changes in the drug market, meanwhile, are undermining the idea that the problem can be dealt with only at an international level. Synthetic drugs, such as amphetamines and Ecstasy, are now more widely used than cocaine and heroin. Scientists dream up new “highs”, while the law lags. As a result, the neat distinction between “consumer”, “supply” and “transit” countries has broken down: the United States and Europe are big producers of cannabis and synthetics, while Brazil, formerly a “transit” country, is now the world’s second-biggest consumer of cocaine. That is leading to experimentation with drug policy at a national and state level.

The Economist has long argued that prohibition is illiberal in principle and harmful in practice, and that the least-bad way of dealing with drugs is to legalise and regulate their production and consumption. But we recognise that it takes a brave politician to face down the moral panic that surrounds the issue. This new thinking, though limited, is therefore welcome. Legalising consumption allows drug use and addiction (by no means the same thing) to be treated as the public-health issues they are. That in turn means applying the principle of harm reduction, for example by providing clean needles to addicts to prevent the spread of HIV.

But decriminalising consumption does nothing to break the grip of gangsters over the drug business. For that to happen, production and distribution also need to be legalised. That is why the experiment under way in the United States is so important. Colorado and Washington now have the chance to create a legal but regulated market in marijuana, similar to those for tobacco or alcohol. Their referendums approved sales of drugs through regulated outlets only, and not to minors. The states now need to design a way of taxing cannabis that discourages consumption while avoiding the creation of a black market.

This experiment has three potential benefits. It should help to determine whether legalisation boosts drug use. It will undermine Mexican drug gangs, which earn perhaps $2 billion a year from cannabis exports to America. And it might provide a model for regulating other, harder, drugs.

The feds should stand back

A threat hangs over the scheme: in 2005 the Supreme Court upheld the federal ban on marijuana for medical use, even in states where this was legal, because of the risk that the drug would leak to other states. The danger of leakage will increase once this experiment gets under way. So it is encouraging that Barack Obama has said that he does not see prosecution of pot smokers in Colorado and Washington as a “top priority”, which means that he plans to do nothing for the moment. Since most of the benefits of legalisation will take a while to show up, it is to be hoped that he will hold his nerve.

One immediate consequence is that the United States will be in breach of the UN Convention. Good. It should now join Latin American governments in an effort to reform that outdated document to allow signatories room to experiment. Imposing a failed policy on everybody benefits nobody.