For years, the UK government has consistently used a myth to stave off any bold innovations in our failing drug policy. It has predicted that liberalisation would inevitably lead to higher rates of drug use and related harms. The example of Portugal seriously challenges this argument.

The idea that decriminalisation increases drug problems was already on shaky ground. Several countries and US states have reduced or eliminated criminal penalties for drug possession. They do not generally have higher rates of drug use than their neighbours. The rate of cannabis use in the Netherlands, for example, is lower than in the UK, even though the Dutch decriminalised the use of cannabis in 1976. The Portuguese experience again shows that there is no necessary link between the severity of sanctions and rates of drug use. There have been some increases reported in drug use by Portuguese adults since 2001, but these are no greater than those seen in other southern European countries. More important are rates of drug use among children, as they are more vulnerable to the harmful effects of illicit drugs. In Portugal, these rates have fallen since decriminalisation.

As fewer people were arrested for drug offences, the prison population fell. So did drug use and HIV among prisoners.

Portugal has also expanded its treatment programmes, including the use of prescribed methadone. This would have reduced deaths and infections even without decriminalisation, as it has elsewhere.

There are limits to the ability of decriminalisation to deal with harm related to illicit drug markets. The policy has not cleared the dealers off the cobbled streets of Lisbon's Bairro Alto. It cannot eliminate the violence related to the unregulated economy of drug supply. Since decriminalisation, Portugal's murder rate has increased, but it is not known how much of this is linked to the drugs trade. The answer could be to take the business out of the hands of violent criminals by handing it to tightly regulated legal organisations, as has been suggested by the Transform Drug Policy Foundation and others.

The potential benefits and costs of drug policy innovations will remain incalculable as long as governments refuse to implement and research them. The Portuguese began their apparently beneficial policy only after commissioning research from respected academics. In the UK last autumn, we saw the removal of the government's leading scientific adviser, Professor David Nutt, for daring to tell the truth about its policies on cannabis and ecstasy. Labour and the Conservatives colluded in this denial of the evidence. They are now conspiring to block the creation of more effective, evidence-based policies on alcohol. They have argued against the introduction of a minimum unit price in Scotland on the basis that it is experimental. But without experimentation, how can we create more effective policies?

Politicians usually only suggest decriminalisation when they are either on the verge of retirement or at the fringes of power. As a young MP, David Cameron himself acknowledged the failures of current policies. He supported fresh thinking on liberalisation, heroin prescription and drug consumption rooms. The Portuguese example suggests he should have the courage of his earlier convictions, now that he is in a position to enact them.

Alex Stevens is professor of criminal justice at the University of Kent and the author of Drugs, Crime and Public Health (Routledge, 2011)