Three United Nations treaties, the oldest from 1961, seek to "advance the health and welfare of mankind" by prohibiting the non-medical use of some drugs. To this end, countries criminalise producers, traffickers, dealers, and users at an annual cost of at least $100bn. But the effectiveness of prohibition laws must be judged on outcomes. And too often the war on drugs plays out as a war on the millions of people who use drugs, and disproportionately on people who are poor or from ethnic minorities and on women.