It's pleasing to see, in the storm of commentary over Professor David Nutt's sacking as the government's chief drugs adviser, that everyone outside politics now recognises the importance of scientific evidence in devising laws. But a strange reasoning twitch has appeared, in the arguments of politicians and rightwing commentators. Science can tell us about the molecules, they say, about their effect on the body and the risks. But policy is separate: a matter for judgment calls on social and ethical issues. Only politicians, they say, can determine the correct way to send out a clear message to the public. It is not a matter for science.

This is wrong. Alongside research into the risks of drugs, lots of work has also been done on the deterrent impact of different laws, classifications and levels of enforcement. As every piece of research has its own imperfections (and nobody has yet conducted a randomised controlled trial on drugs policy) you can make your own mind up about whether you find the results compelling.

One strategy is to compare different countries. A World Health Organisation study from 2008, published in the academic journal PLOS Medicine, compared drug use and enforcement regimes around the world. It was clear: "Globally, drug use is not distributed evenly and is not simply related to drug policy, since countries with stringent user-level illegal drug policies did not have lower levels of use than countries with liberal ones."

Alternatively, you can compare drug use between states within one country, if they have very different enforcement regimes, as when parts of the US liberalised their laws a few decades ago.

In 1976 Stuart and colleagues found that cannabis use in Ann Arbor, Michigan, was not affected by reductions in cannabis penalties, when compared with three neighbouring communities which kept penalties the same.

In 1981 Saveland & Bray looked at national drug use surveys from 1972 to 1977 and found cannabis use was higher in "decriminalised" states, before and after changes in the law. When they looked at rates of change, although cannabis use was increasing everywhere, the most rapid increase was in the states with the most severe penalties.

In the same year, Johnson and colleagues used survey data on high school use and found decriminalisation had no effect on attitudes or beliefs about drugs. These studies are old, but only because the liberalisations they rely on for data happened a long time ago.

Another line of evidence comes from "before and after" studies, when laws are changed. Cannabis use in the UK dropped after cannabis was moved from class B to class C. Prohibition of alcohol in the US from 1920 to 1933 is the most famous example: alcohol use fell dramatically when prohibition began, and the price of alcohol rose to 318% of its previous level. By 1929 this initial impact had begun to wear off and rapidly: alcohol consumption had risen to 70% of pre-prohibition levels, and was still rising when prohibition was repealed, and the price had fallen to 171% of pre-prohibition levels. This reversion to old patterns of use occurred despite escalating spending on enforcement, up 600% over the same period. There are many more examples.

This is not an unresearchable question. There are other factors at play in all of these studies, and if they are not sufficiently rigorous for the government, or a brief informal dip into the literature is not enough, (it shouldn't be) then they should commission more research: because it is a tenet of evidence-based policy that if you discover a gap, you commission work to fill it.

This work is important for one simple reason. If you wish to justify a policy that will plainly increase the harms associated with each individual act of drug use, by creating violent criminal gangs as distributors, driving the sale of contaminated black market drugs, blighting the careers of users caught by the police, criminalising three million people, and so on, then people will reasonably expect, as a trade-off, that you will also provide good quality evidence showing that your policy achieves its stated aim of reducing the overall numbers of people using drugs.