Pure fentanyl is 100 times more potent than morphine, and doses are measured in micrograms instead of milligrams: Unless it’s mixed very precisely and very evenly, the strength of the drug can vary dramatically—dangerously—from one hit to the next. A few years ago, the Midwest and East Coast saw at least 1,000 heroin overdoses after a lab in Toluca, Mexico, produced fentanyl that was then cut into the heroin supply chain, making the heroin much, much stronger—unevenly so.