Fentanyl is a synthetic opioid that’s 50-100 times stronger than the pain reliever morphine. While some people use fentanyl on purpose, many others get it in their systems without their knowledge.

Proponents say the ability to check drugs for the presence of lethal fentanyl may save lives. But critics say the strips enable drug use. By Jan Hoffman Photographs by Desiree Rios “You smoke weed?” ...