It looked like the first drop of rain in the desert of drugs policy. Last week Antonio Maria Costa, the executive director of the UN office on drugs and crime, said what millions of liberal-minded people have been waiting to hear. "Law enforcement should shift its focus from drug users to drug traffickers … people who take drugs need medical help, not criminal retribution." Drug production should remain illegal, possession and use should be decriminalised. Guardian readers toasted him with bumpers of peppermint tea, and, perhaps, a celebratory spliff. I didn't.

I believe that informed adults should be allowed to inflict whatever suffering they wish – on themselves. But we are not entitled to harm other people. I know people who drink fair-trade tea and coffee, shop locally and take cocaine at parties. They are revolting hypocrites.

Every year cocaine causes some 20,000 deaths in Colombia and displaces several hundred thousand people from their homes. Children are blown up by landmines; indigenous people are enslaved; villagers are tortured and killed; rainforests are razed. You'd cause less human suffering if instead of discreetly retiring to the toilet at a media drinks party, you went into the street and mugged someone. But the counter-cultural association appears to insulate people from ethical questions. If commissioning murder, torture, slavery, civil war, corruption and deforestation is not a crime, what is?

I am talking about elective drug use, not addiction. I cannot find comparative figures for the United Kingdom, but in the United States casual users of cocaine outnumber addicts by about 12 to one. I agree that addicts should be helped, not prosecuted. I would like to see a revival of the British programme that was killed by a tabloid witch-hunt in 1971: until then all heroin addicts were entitled to clean, legal supplies administered by doctors. Cocaine addicts should be offered residential detox. But, at the risk of alienating most of the readership of this newspaper, I maintain that while cocaine remains illegal, casual users should remain subject to criminal law. Decriminalisation of the products of crime expands the market for this criminal trade.

We have a choice of two consistent policies. The first is to sustain global prohibition, while helping addicts and prosecuting casual users. This means that the drugs trade will remain the preserve of criminal gangs. It will keep spreading crime and instability around the world, and ensure that narcotics are still cut with contaminants. As Nick Davies argued during his investigation of drugs policy for the Guardian, major seizures raise the price of drugs. Demand among addicts is inelastic, so higher prices mean that they must find more money to buy them. The more drugs the police capture and destroy, the more robberies and muggings addicts will commit.

The other possible policy is to legalise and regulate the global trade. This would undercut the criminal networks and guarantee unadulterated supplies to consumers. There might even be a market for certified fair-trade cocaine.

Costa's new report begins by rejecting this option. If it did otherwise, he would no longer be executive director of the UN office on drugs and crime. The report argues that "any reduction in the cost of drug control … will be offset by much higher expenditure on public health (due to the surge of drug consumption)". It admits that tobacco and alcohol kill more people than illegal drugs, but claims that this is only because fewer illegal drugs are consumed. Strangely however, it fails to supply any evidence to support the claim that narcotics are dangerous. Nor does it distinguish between the effects of drugs themselves and the effects of the adulteration and disease caused by their prohibition.

Why not? Perhaps because the evidence would torpedo the rest of the report. A couple of weeks ago, Ben Goldacre drew attention to the largest study on cocaine ever undertaken, completed by the World Health Organisation in 1995. I've just read it, and this is what it says. "Health problems from the use of legal substances, particularly alcohol and tobacco, are greater than health problems from cocaine use. Few experts describe cocaine as invariably harmful to health. Cocaine-related problems are widely perceived to be more common and more severe for intensive, high-dosage users and very rare and much less severe for occasional, low-dosage users … occasional cocaine use does not typically lead to severe or even minor physical or social problems." This study was suppressed by the WHO after threats of an economic embargo by the Clinton government. Drugs policy in most nations is a matter of religion, not science.

The same goes for heroin. The biggest study of opiate use ever conducted (at Philadelphia general hospital) found that addicts suffered no physical harm, even though some of them had been taking heroin for 20 years. The devastating health effects of heroin use are caused by adulterants and the lifestyles of people forced to live outside the law. Like cocaine, heroin is addictive; but unlike cocaine, the only consequence of its addiction appears to be … addiction.

Costa's half-measure, in other words, gives us the worst of both worlds: more murder, more destruction, more muggings, more adulteration. Another way of putting it is this: you will, if Costa's proposal is adopted, be permitted without fear of prosecution to inject yourself with heroin cut with drain cleaner and brick dust, sold illegally and soaked in blood; but not with clean and legal supplies.

His report does raise one good argument, however. At present the trade in class A drugs is concentrated in the rich nations. If it were legalised, we could cope. The use of drugs is likely to rise, but governments could use the extra taxes to help people tackle addiction. But because the wholesale price would collapse with legalisation, these drugs would for the first time become widely available in poorer nations, which are easier for companies to exploit (as tobacco and alcohol firms have found) and which are less able to regulate, raise taxes or pick up the pieces. The widespread use of cocaine or heroin in the poor world could cause serious social problems: I've seen, for example, how a weaker drug – khat – seems to dominate life in Somali-speaking regions of Africa. "The universal ban on illicit drugs," the UN argues, "provides a great deal of protection to developing countries".

So Costa's office has produced a study comparing the global costs of prohibition with the global costs of legalisation, allowing us to see whether the current policy (murder, corruption, war, adulteration) causes less misery than the alternative (widespread addiction in poorer nations)? The hell it has. Even to raise the possibility of such research would be to invite the testerics in Congress to shut off the UN's funding. The drug charity Transform has addressed this question, but only for the UK, where the results are clear-cut: prohibition is the worse option. As far as I can discover, no one has attempted a global study. Until that happens, Costa's opinions on this issue are worth as much as mine or anyone else's: nothing at all.