US and European authorities are battling a new enemy in the war against opioids. Nitazenes are a class of synthetic drugs 40 times more potent than fentanyl that have caused hundreds of confirmed deaths across Europe and the US since appearing on the radar of law enforcement agencies in 2019. However, this figure is likely to be a significant undercount.

Nitazenes were first synthesized in the 1950s by CIBA Aktiengesellschaft, an Austrian chemical company, which created several chemically related molecules with different levels of pain-killing potency. However, their use as painkillers never took off. As well as being highly addictive, nitazenes can cause respiratory depression, a dangerous condition where breathing becomes too shallow to replenish oxygen in the blood. These drugs were therefore largely unheard of for decades until they appeared in the illegal market.

It’s difficult to say exactly when nitazenes started being commonly sold as street drugs—identifying them requires specific tests that are not routinely performed—but law enforcement agencies started noticing them about six years ago. A shipment of one type of these synthesized molecules—isotonitazene—was intercep...