Sam Bowman is Research Director of the Adam Smith Institute

Although I know her intentions are good, there are many weak points in Kathy Gyngell’s recent piece on drugs legalisation.

No, “ageing libertarians” are not pushing legalization on abstemious youngsters – in the US, at least, a recent poll found that 67 per cent of under-30s supported legalisation, compared to 58 per cent of the population in general.

No, it is not legitimate to dismiss inconvenient research because it comes from “pro-drugs lobby groups”. Nobody should dismiss Kathy’s arguments simply because she is anti-drugs either. We should all play the ball, not the man.

And no, it’s not good enough to say that drug laws don’t matter because drug use is ‘marginal’ compared to alcohol and tobacco use. In the 12 months ending March 2013, there were around 87,000 convictions for drug offences. If our laws are unjust, then roughly 87,000 people are being wrongly convicted every year. Around 70 per cent of drug offenses relate to cannabis possession.

The best case Kathy could make would be roughly this: that drugs like cannabis, MDMA and cocaine are extremely harmful to their users, so it is worth harming several thousand of them by imprisoning them and giving criminal records to hundreds of thousands of others, as we currently do, because legalisation would lead to so much more use that the extra harm would be even worse than the status quo.

We don’t ban skydiving, horseriding or rugby, even though those are all at least as risky as smoking cannabis. The argument usually has to focus on the harm caused to children, because most people now reckon that, at least to some extent, adults should be able to take risks with their bodies if they want to.

Unfortunately Kathy does not make this argument: she totally ignores the harm caused by criminal convictions to those unlucky enough to get caught. Drugs may cause harm, but so do drug laws.

There are a few strands to the stronger parts of Kathy’s argument, which deserve to be considered separately. She is right to highlight the fact that Lambeth’s experiment with cannabis decriminalization basically failed: there was a big leap in the number of hospital admissions relating to class-A drug use. The authors of the paper she cites suggest that the rise in cannabis use went alongside a rise in class-A drug use, causing more hospital admissions overall.

But the “Brixton experiment” may just show us that fiddling around the edges isn’t a good way to improve drug policy. Only cannabis was affected, and it was decriminalized, not legalised. That means that the supply was still in the hands of criminals and users had no guarantee that what they were taking was pure. Indeed, this is one of the major reasons why we “ageing libertarians” are so keen on legalisation as opposed to decriminalization – suppliers need to be out in the open and answerable to the law if they sell a dodgy product.

On the Coloradan experience, Kathy makes a few errors – medical marijuana was legalized there by Amendment 20 in 2000, not 2009 (2009 was when a Board of Health hearing clarified the position of marijuana dispensaries), and total school expulsions are not up by one-third, as she claims: rather, drug-related suspensions and expulsions are up by a third.

Aside from these, she uses well-chosen anecdotes to claim that medical marijuana expansion has driven an increase in teen marijuana use. But what she doesn’t mention is that teen marijuana use in Colorado actually fell between 2009 and 2011, from 25 per cent to 22 per cent, despite the fact the overall trend was up, with use rising from 21 per cent to 23 per cent nationwide.

Other evidence contradict’s Kathy’s version of events: a 2012 study by researchers at the Universities of Colorado and Oregon and Montana State University concluded that there was no evidence to link the legalization of medical marijuana to increased use of the drug among high school students.

Kathy does not explain why she (presumably) thinks tobacco and alcohol should remain legal when both of those substances are at least as harmful as cannabis. Cannabis smokers are 2.6 times more likely than non-smokers to have a psychotic-like experience; but tobacco smokers are 20 times more likely to contract lung cancer than non-smokers. It’s hard to avoid the thought that some people only care about those prohibitions that affect them.

The question Kathy never asks about drugs is: what’s the alternative? Yes, drug driving rates may rise, but this rise seems to go alongside a fall in drunk driving. Medical marijuana laws are associated with a decline in traffic fatalities, possibly because reckless drivers smoke instead of drinking, and cannabis has less of an impact on driving than alcohol.  More generally, if cannabis is substituted for more-harmful substances like alcohol or harder drugs, the fact that it is harmful in an absolute sense is beside the point.

Of course, the debate isn’t just about harm reduction. Adults having control over their bodies, free from the interference of other people, is a good thing in and of itself. And while drugs may ‘enslave’ some addicts, to generalize this to claim that all drugs enslave all or even most drug users is ludicrous. Those addicts who really do need help would be able to ask for it far more easily in a world where they don’t need to fear imprisonment.

Kathy is right that legalisation will almost certainly mean more drug use. But prohibition means putting thousands of people in jail, giving criminal records to hundreds of thousands of others, and exposing drug users to the risk that they may take something truly lethal when they think they’re taking some ecstasy. Kathy may be happy to throw other people’s children in jail for trying cannabis. I can only be thankful that the tide of public opinion is against her.