Heroin screws you up. Everybody knows that. At least, everybody who was alive and literate in the mid-1980s should know that, because Mrs Thatcher’s government slapped the slogan on posters all over the UK, illustrated with the emaciated figure of an adolescent victim and a list of his heroin-related screw-ups: skin infections, liver complaints, blood diseases, mental problems.

It was all a lie.

It wasn’t Mrs Thatcher’s own personal lie. The bizarre campaign of official misinformation about heroin goes back to the 1920s, specifically to the congressional committee in Washington DC, which met in April 1924 to consider the facts about heroin. Solemnly, they recorded the views of physicians of the utmost fame that heroin obliterated the herd instinct, strengthened the muscular reflexes of criminals and caused insanity. Emboldened by these exciting fantasies, the committee decided that heroin must be banned not only in the United States but also around the entire planet. Within two months, their proposal had been converted into law, and the War Against Drugs was born.

The truth about heroin is that it is a benign drug. It is addictive – and that is a very good reason not to use it – but the only significant screw-up it inflicts on the physical, mental and moral condition of its users is constipation. Contrary to many decades of official claims, it is rather difficult to kill yourself with heroin: the gap between a therapeutic and a fatal dose is far wider than it is, for example, with paracetamol. The truth, which for decades has scarcely dared to speak its name, is that all of the illness, misery, death and crime associated with heroin are, in fact, the effect not of the drug itself but of the black market on which it is sold as a result of this war against drugs.

Black market heroin becomes poisonous and dangerous because criminals cut it with pollutants to increase their profit. Black market addicts contract diseases, because they use dirty injecting equipment. Black market users overdose accidentally, because they have no idea of the purity of the batch they are using. Black market addicts are thin, not because the drug makes them thin, but because they have to give all their money to dealers. Black market addicts commit crime and sell themselves for sex, not because the drug makes them immoral, but because they have no other way to fund their habit.