Dubbed the “killer opioid,” fentanyl can come in liquid or powder form, or made to look like a prescription pill. It is known to be extremely potent: 50 times stronger than heroin and 100 times stronger than morphine. That means even a very small amount can be lethal. During an overdose, people’s breathing may slow or stop altogether.

It is impossible to know just by looking at a drug if it has been tainted with fentanyl. That is why health experts and harm reduction advocates have been pushing for the wider distribution of fentanyl test strips, which can help users detect whether fentanyl is present in their supply. And while some states ban these test strips  —  based on the argument that this instigates drug use — California promotes and distributes them for free.