I was asked to speak on a panel this morning to give the case for cannabis legalisation, and found it a useful exercise in tidying and focusing my thoughts. My overarching point was that, if it does happen, it’s probably going to be because of a fundamental shift in public opinion of the kind that’s taking place in some parts of the United States, so at this stage it’s most useful to consider how we might want to approach decriminalisation or full legalisation. Still, the broad case for cannabis legalisation is worth making, and I tried to do so.

Fundamentally: because it’s an enjoyable consumer product that causes less harm than many existing legal products that nobody sane thinks we should make illegal, and the harms that it does cause are best dealt with by cannabis being a legal product.

2.1 million people use cannabis a year, 6.5% of the UK population (according to the Home Office Crime Survey for England and Wales). That’s an awful lot of people who are de facto criminals, and even a broad estimate of the proportion of ‘problem users’ is relatively small – and there’s not much reason to think that the drug being illegal is good for problem users anyway.

The law isn’t predictably or consistently enforced – arrests for possession of cannabis in England and Wales have dropped by 46 percent since 2010. Cautions dropped by 48 percent and charges fell by 33 percent as of 2016. This is mostly because funding has been cut, targets have changed and stop and search is being used less. Other than possession with intent to supply, it's not a priority for most police forces. Having a law on the statute books that is capriciously enforced makes an ass of the law in general.

Prohibition increases the supply of stronger and potentially more harmful types of cannabis – as Sam Dumitriu put it, prohibition created skunk. Suppose we treated alcohol the same way we treat cannabis: very quickly the most widely available stuff would be strong spirits, because they are a much more concentrated way of delivering alcohol than beer or wine. By imposing effectively a blanket additional cost on drug supply, drug prohibition encourages the most concentrated drugs to dominate the market. Most cannabis users and would-be users that I am aware of would prefer to buy less strong stuff, but find it difficult – because the market is not being allowed to function properly.

Most revenues from drug supply goes to criminal gangs, creating financing for other (more harmful) activities & violent side-effects such as turf wars. Taking the cannabis supply chain out of the hands of people willing to kill and putting it into the hands of organisations like Boots would reduce violence, and make the suppliers legally accountable to their customers in the event of fraud.

Legalisation could produce benefits to the government of £750m–£1.05bn in tax revenues and lower criminal justice costs. I’m not too excited about this, because a revenue-hungry government might well put the tax up to a level that’s so high that black market cannabis is still attractive to users. This has been a problem in Colorado – regular users are still buying from their (untaxed) black market dealers, who can undercut the legal market, so some of the benefits we hoped for haven’t materialised.

The public probably isn’t as opposed as you think. Polling results differ wildly depending on how we frame the question. Polls that stress the regulated nature of a legal market produce majorities or near-majorities in favour of legalisation or decriminalisation; polls that just ask the question straight show majorities or near-majorities against. That suggests to me that opposition is fairly soft and the big question is how reform happens and how the debate is framed. In the US, the polling evidence suggests that people have very specific things in mind (street consumption, teenagers’ access to the drug), which we might be able to craft a regulatory framework to avoid.