There’s a new beast on the streets — and it’s nothing to bark at. A powerful animal tranquilizer is increasingly saturating the US drug supply, fueling mass overdose outbreaks in hot spots like Chicago and Philadelphia. It was detected in New York last year, setting off alarm bells among public health officials. They warn that while its effects can mimic opioids such as fentanyl, there’s a dangerous catch: It doesn’t respond to conventional overdose-reversal treatments. The drug, medetomidine, is a sedative used by veterinarians to relax muscles and relieve pain, often during surgery. It works by slowing the release of...