Judith Harvey is recently back from Colombia, which is slowly recovering from decades of violence brought about by its illegal manufacture and export of cocaine.

Twenty-five years ago Medellín in Colombia was the murder capital of the world. The city was in the grip of Pablo Escobar’s drugs cartel. His teenage sicarios – hitmen – killed without a second thought to maintain his control over the politicians, social organisations and the rival Cali cartel. During a brief venture into politics, Escobar initiated a housing programme and the poor loved him for it, but there was hardly a mother who hadn’t lost a husband, brothers, sons, in the narcotraficantes’ guerilla warfare.

Then, in 1993, Pablo Escobar was shot dead in a police raid.

Drugs are a traditional part of Latin American culture. 2,500 miles and 400 years away in Potosí, Bolivia, more than 4000m up in the Andes, men and boys mined silver from the Cerro Rico, the rich mountain, to fill the king of Spain’s coffers. The miners chewed coca leaves to help them survive the long arduous hours of work and the hunger and the pain. Silver is still mined there, and they still chew coca.

Coca numbs a brutal and exhausting life but it doesn’t give users a high. In most traditional communities the use of hallucinogens is privileged to mystics. The counterculture picked them up and the trickle of strongly psychoactive and very addictive drugs into the rich world has become a tsunami stimulated by millions of users who would seem to have every opportunity to live fulfilling lives without drugs.

Prescribed medicines like fentanyl and ketamine migrate into recreational use. Overprescribing of opioids in the USA has led to 150 deaths every day from overdoses – the oxycontin scandal is now in all the papers, and knowing how the Sackler family made its millions, their generosity in endowing art galleries such as London’s Royal Academy takes on a different colour. Chemists in jungles and laboratories manufacture stronger versions of traditional drugs: coca gives rise to cocaine, and cocaine to crack. Cannabis gives way to skunk and spice.

Drugs are ever more readily available. The cost, personal and to society, is massive, and for decades governments have attempted to stem the tide.

You can try to dissuade people from using drugs. But did Nancy Reagan’s Just Say No campaign dissuade anyone? Since Trump doesn’t go in for evidence-based policy-making he will probably ignore the Singapore experience that the death penalty doesn’t reduce the use of drugs. Using drugs is something humans do.

You can make drugs illegal. But it doesn’t work, as prohibition of alcohol in the USA demonstrated. The result was bootlegged spirits and gangland violence. In Britain upgrading cannabis to a class B drug spawned spice: more profit for the same risk. Deaths from legal highs have increased since head shops were driven underground. Drugs are rife in our society. Especially in our prisons.

You can try to eliminate drugs at source. The USA has pumped millions of dollars into eradication programmes in Colombia. The result: coca farmers have been deprived of their livelihood, the herbicides have caused widespread environmental damage and health problems, the narcotraficantes took over and disaffected militias of every persuasion used the drug trade to finance their operations.

The ‘War on Drugs’ is another conflict we can’t win. The alternative is to decriminalise drugs. That means not just private use, but supply, which is where the criminals operate. But social and political attitudes are a big obstacle. I remember an academic who lectured us medical students on the dangers of weed, even though his lifetime of research had failed to demonstrate that cannabis causes significant harm.

Politicians too put their heads in the sand. The 2014 report to the All Party Parliamentary Group on Drug Policy Reform concluded that legalisation in countries such...