On a morning in November 2023 Eamon Keenan, a psychiatrist who runs addiction services at Ireland’s state-funded health-care provider, received a worrying phone call. “People in homeless accommodation and hospitals are collapsing,” he recalls being told. It was the start of a bleak few weeks. In Dublin and Cork, the country’s biggest cities, 77 people would end up overdosing. The initial suspect was dodgy heroin, but laboratory analysis revealed a dangerous new class of drugs—nitazenes. Since then, these have been detected everywhere from Freetown in Sierra Leone to Sydney in Australia.