In areas of moral and political conflict people will always behave badly with evidence, so the war on drugs is a consistent source of entertainment. We have already seen how cannabis being "25 times stronger" was a fantasy, how drugs-­related deaths were quietly dropped from the measures for drugs policy, and how a trivial pile of poppies was presented by the government as a serious dent in the Taliban's heroin revenue.

The Commons home affairs select committee is looking at the best way to deal with cocaine. You may wonder why they're bothering. When the Advisory Council for the Misuse of Drugs looked at the evidence on the reclassification of cannabis it was ignored. When Professor David Nutt, the new head of the advisory council, wrote a scientific paper on the relatively modest risks of MDMA (the active ingredient in the club drug ecstasy) he was attacked by the home secretary, Jacqui Smith .

In the case of cocaine there is an even more striking precedent for evidence being ignored: the World Health Organisation (WHO) conducted what is probably the largest ever study of global use. In March 1995 they released a briefing kit which summarised their conclusions, with some tantalising bullet points.

"Health problems from the use of legal substances, particularly alcohol and tobacco, are greater than health problems from cocaine use," they said. "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."

The full report – which has never been published – was extremely critical of most US policies. It suggested that supply reduction and law enforcement strategies have failed, and that options such as decriminalisation might be explored, flagging up such programmes in Australia, Bolivia, Canada and Colombia. "Approaches which over-emphasise punitive drug control measures may actually contribute to the development of heath-related problems," it said, before committing heresy by recommending research into the adverse consequences of prohibition, and discussing "harm reduction" strategies.

"An increase in the adoption of responses such as education, treatment and rehabilitation programmes," it said, "is a desirable counterbalance to the over-reliance on law enforcement."

It singled out anti-drug adverts based on fear. "Most programmes do not prevent myths, but perpetuate stereotypes and misinform the general public.

"Such programmes rely on sensationalised, exaggerated statements about cocaine which misinform about patterns of use, stigmatise users, and destroy the educator's credibility."

It also dared to challenge the prevailing policy view that all drug use is harmful misuse. "An enormous variety was found in the types of people who use cocaine, the amount of drug used, the frequency of use, the duration and intensity of use, the reasons for using and any associated problems."

Experimental and occasional use were by far the most common types of use, it said, and compulsive or dysfunctional use, though worthy of close attention, were much less common.

It then descended into outright heresy. "Occasional cocaine use does not typically lead to severe or even minor physical or social problems … a minority of people … use casually for a short or long period, and suffer little or no negative consequences."

And finally: "Use of coca leaves appears to have no negative health effects and has positive, therapeutic, sacred and social functions for indigenous Andean populations."

At the point where mild cocaine use was described in positive tones the Americans presumably blew some kind of outrage fuse. This report was never published because the US representative to the WHO threatened to withdraw US funding for all its research projects and interventions unless the organisation "dissociated itself from the study" and cancelled publication. According to the WHO this document does not exist, (although you can read a leaked copy at www.tdpf.org.uk/WHOleaked.pdf).

Drugs show the classic problem for evidence-based social policy. It may well be that prohibition, and distribution of drugs by criminals, gives worse results for the outcomes we think are important, such as harm to the user and to communities through crime. But equally, we may tolerate these outcomes, because we decide it is more important that we declare ourselves to disapprove of drug use. It's okay to do that. You can have policies that go against your stated outcomes, for moral or political reasons: but that doesn't mean you can hide the evidence.