Cannabis can be harmful; it is naïve to suggest otherwise. But given the reality of the drug’s widespread use, the more interesting and important question is how best to minimize these harms. Over the past century, the chosen solution throughout most of the world has been to prohibit cannabis and criminalize those who possess or use it. Indeed, it has long been a central tenet of drug policy-makers that this punitive approach has a deterrent effect.

At first glance, this seems like a reasonable, common-sense assumption – but what does the evidence actually show? Do harsh cannabis laws mean fewer cannabis users?

Before answering this question, it is important to emphasize that levels of drug use cannot simply be equated with levels of harm. The United Nations Office on Drugs and Crime, for example, estimates that roughly 90% of illicit drug use is “non-problematic.” Hence deterring people from using drugs does not automatically mean saving them from harm. In fact, it is entirely conceivable that, under certain conditions, allowing adults to consume some drugs legally – but more safely and responsibly – may increase use but decrease overall harm.  

Bearing that caveat in mind, the available evidence suggests that criminal (or other) penalties have, at best, a marginal impact in deterring people from using cannabis. The European Monitoring Centre for Drugs and Drug Addiction looked at this issue specifically in relation to cannabis. Researchers there examined data from nine European countries, to test what they call the “legal impact hypothesis” – essentially the theory that increased penalties will lead to a fall in drug use and reduced penalties will lead to a rise in drug use.

They concluded: “in this 10-year period, for the countries in question, no simple association can be observed between legal changes and cannabis use prevalence.” In other words, the fact that some countries’ cannabis laws became harsher and some became more lenient had no discernible effect on the number of people using the drug.

This (lack of an) effect was observed when the penalties for cannabis possession were reduced in England and Wales in 2004. The drug was downgraded from a class B to a class C substance, and yet the prevalence of cannabis use, already declining since 2001, has continued to decline at about the same rate ever since (even after being upgraded back to class B drug again in 2009).

And it is not just legal changes within countries that seem to make little difference. Comparisons of different countries’ approaches to drugs and their respective levels of drug use also produces the same result. A large-scale study using World Health Organization data from 17 countries found: “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.”

This disconnect between drug law enforcement and prevalence of use may partly be explained by psychological factors. For people thinking of using drugs, pleasure is typically a more important consideration than the potential penalties they might face. And these penalties are even less likely to be at the forefront of users’ minds given that, although many countries certainly do arrest large numbers of drug users, these people are, in relative terms, only a small proportion of the total drug-using population. The practical difficulties in detecting small quantities of banned substances mean the chance of arrest for personal drug possession is, for most people, relatively slim.

So although politicians often defend punitive drug laws on the grounds that they “send a message” about the unacceptability and dangers of drug use, there is little evidence that this message gets through to users. Rates of drug use are more likely to rise and fall in line with broader cultural, social or economic trends. Ultimately, the criminal justice system appears to be a poor vehicle for public health advice.