A recent study found that, in US states where medicinal marijuana has been legalised, alcohol usage has fallen significantly. Given that alcohol is known to be far more harmful than cannabis, this rips away one of the last possible arguments for keeping weed illegal.

Advocates for the continuation of cannabis prohibition have argued that to legalise weed would be to introduce new health hazards on top of the existing ones posed by alcohol, and simply make things even worse. If people smoked cannabis as well as continuing to drink at the same level, this might be true. But until now, we haven’t known for sure how people’s drinking habits might be affected when cannabis was legalised.

Now, with the majority of US states allowing weed consumption – for medical use at least – we know that for some people, cannabis replaces alcohol as the drug of choice. The legalisation of weed appears to be linked with a 15% decline in alcohol consumption.

The toxicity of cannabis is so low that it’s technically impossible to overdose on it. An addiction blog states (unnecessarily) that while it is “impossible to have a toxic overdose of marijuana, it is quite possible to consume more weed than necessary” (the same advice might be applied to potatoes). Meanwhile in 2017, alcohol was directly linked to over 7,000 British deaths. So any shift from alcohol to ganja is good news for public health.

It has never been especially clear why cannabis, a drug used by humans for thousands of years, was made illegal in the first place. The first ban by westerners was probably carried out in the early nineteenth century by Napoleon, who had invaded Egypt, and who was concerned about his troops’ enjoyment of the local hashish. Following that ban, the French and British empires increasingly tried to restrict the drug, and other governments around the globe followed suit. In Britain, weed usage was fairly low until recent decade, but the UK government banned it anyway in 1928. The United States followed with a nationwide ban in 1937.

There are various theories as to why America did this. Some people believe that weed, being a drug favoured (at that time) mostly by black and Latino Americans, was outlawed because of racism. These arguments are fairly weak, however. Puritans have consistently sought to outlaw all drugs, regardless of who uses them. It’s true that cannabis use was favoured by racial minorities, but drugs like LSD, ecstasy, heroin and methamphetamine have also been banned, despite being heavily used by white people. Even alcohol, the white man’s drug of choice, was prohibited in the US for a while.

More likely, cannabis was simply prohibited worldwide out of fear and ignorance. Once the anti-cannabis panic had taken root, it would have taken a foolhardy politician to stand against the tide. Even the United Nations declared, in 1961, its intention to stamp out recreational cannabis use worldwide within 25 years (obviously, that didn’t quite go to plan).

The prohibition tide began to turn in the 1970s in Amsterdam, when “coffeeshops” began to be licensed as smoking venues. Weed still wasn’t legal in the Netherlands, but its use was decriminalised. Ever since, the cultural pressure to legalise cannabis has grown. Peter Tosh’s classic 1976 hit Legalize It was one of the early pro-cannabis protest songs, and was followed by many more, especially in the reggae and hip hop scenes.

Once Amsterdam could provide a safe space for the cannabis industry to meet, it became a hub where growers could get together to exchange growing tips and swap seeds. Two annual competitions, the Cannabis Cup and the more US-centric High Times Cup, arose to celebrate the latest breeds, and the winning varieties would take pride of place on coffeeshop menus. Legalisation (or decriminalisation) in America (as well as Canada, Uruguay, Jamaica and other places) has led to huge advances in cannabis growing technology, as well as new ways to consume the drug. Many health-conscious Americans have moved away from smoking weed towards vaping, new forms of concentrates, and edibles ranging from chocolates to smoothies.

Although the legal marijuana industry is still young, the legalisation case is now turning from “but can we risk legalising it?” to “can we risk keeping it illegal?” Any harms caused by a rise in cannabis use are easily offset by a decline in alcohol consumption.

In academic language, the decline in US booze sales means that cannabis is “substitutable” for alcohol, and this effect may also apply to other drugs. Anyone who has been on the clubbing scene will know that ecstasy users tend to drink water or soft drinks rather than alcohol. Although there are risks associated with taking ecstasy (there were 56 related deaths in 2017), these are far lower than drink-related risks. Hallucinogens such as magic mushrooms and LSD carry an ever lower risk, and may replace booze for some people, if they were legalised. It may be that legalising a dozen or so relatively safe drugs could reap huge health rewards by replacing alcohol as the drug of choice.

Until recently, drug legalisation was presented as a foolhardy idea. Now it is becoming fashionable to state that “we have lost the war on drugs”, and to look for alternative ways of dealing with drug problems. It seems almost mad now that alcohol, one of the most dangerous drugs on the market, is also one of the only legal ways to get high. We can’t stop people wanting to take drugs; but we can allow them to choose safer ways of doing so.

Jerry Barnett is a technologist, author and civil liberties campaigner. He runs the Sex & Censorship blog, and his book Porn Panic is available from all good bookshops.