THE roadside billboards in some American towns do not advertise fast-food chains or home insurance. Instead, they tell people what to do in case of a drug overdose. Deaths in America from opioids, pain-relieving drugs that include both prescription painkillers such as OxyContin and illegal ones such as heroin, have almost quadrupled over the past two decades. In some states the share of babies who are born with withdrawal symptoms has increased by 300% since 1999; at least 8,000 were born suffering from them in 2013. Each day 91 Americans die from an opioid overdose.

Much of this catastrophe stems from the over-prescription of legal painkillers. In 2015 some 650,000 prescriptions were handed out on an average day. But when prescriptions end, addicts sometimes turn to illicit substances. The latest one that worries experts is a synthetic opioid called fentanyl, which is around 50 times more potent than heroin and 100 times more powerful than morphine. Most of the fentanyl making its way to America has been made, often legally, in factories in China before being shipped to criminal networks in Mexico and Canada and then smuggled over the border. Thousands of Americans have died from using fentanyl since 2013.

In the face of such numbers, it is always tempting to reach for the comfort blanket of prohibition. The Trump administration is taking a hawkish line on drugs of all kinds. Jeff Sessions, the attorney-general, opposes the legalisation of cannabis. On May 12th he appeared to reverse years of sensible policy that sought to reduce punishment for non-violent drug crimes by instructing prosecutors to pursue the “most serious, readily provable” offences. Efforts are being made to restrict synthetics. In March the government in China, under pressure from America and the UN, agreed to make four variants of fentanyl illegal. Yet such plans will do little to stop the opioid crisis or to curb the threat from fentanyl.

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That is partly because the crisis is too far advanced for criminalisation to work as a deterrent. The country has at least 2m opioid addicts. They need treatment and safer places to take drugs. The health-care bill passed by the House of Representatives this month heads in the opposite direction. Its proposals would cut spending and reduce access to medicines.

But prohibition is futile for more profound reasons, too. An iron law of drugs markets, whether for painkilling opioids or recreational highs, is that demand creates supply and just as much as vice versa. Fentanyl is particularly attractive to criminals. Because it is so potent, with only 2mg of the stuff enough to cause an overdose, it is easy to hide in letters and small packages that are sent by post. The rewards are enormous: 1kg of fentanyl costs around $4,000 to buy from China and yields profits of $1.6m on the streets. By contrast, 1kg of heroin costs around $6,000 but is worth a few hundred thousand dollars.

Fentanyl, and its variants, are among hundreds of new synthetic drugs that have flooded the illicit-drugs market over the past decade (see article). New drugs have been emerging at the rate of one a week; in 2012-17, 20 new fentanyl analogues appeared. A market this protean cannot be erased. Crack down in China, and laboratories will appear in Mexico; already some have opened there. Ban one substance, and another will appear. Whack every mole, as Britain has attempted with a law that prohibits any new drug that has a psychoactive effect, and substances get pushed from shops to the internet.

Banning drugs is not just ineffective, it is also counterproductive. Fentanyl is a nasty substance, but prohibiting all illicit drugs, whether they are new or established, prevents the research that could distinguish between those which are more and less harmful. It also leads to topsy-turvy outcomes. Marijuana, which cannot lead to overdoses and which can be used as an effective pain-relief medicine, is classified by the federal authorities in America as a more dangerous drug than fentanyl, which is used in very controlled doses by cancer patients and abused fatally across the country.

It takes guts to legalise drugs when so many are dying from them. But it is better that addicts take safe doses of familiar substances under sanitary conditions than for them to risk their lives enriching criminals. Switzerland followed the legalisation path after a heroin epidemic in the 1980s, treating drugs as a public-health problem. Since then drug-taking and drug-related deaths have fallen. America should follow suit.