Medetomidine is an animal tranquilizer that is frequently mixed with fentanyl.
The San Francisco Department of Public Health says at this time the drug is not widespread, but it is concerning.
MORE: Fentanyl laced with animal tranquilizer leading to overdoses across US; supervisor urges SF to test
It is prevalent on the East Coast and has resulted in a spike of overdoses.
The Chronicle reports the person who overdosed was a 46-year-old woman who had multiple drugs in her system.