Use of the veterinary drug xylazine is having a profound impact on the health of those who use illicit substances across the United States. Originally developed by the pharmaceutical company Bayer in 1962 as a large animal sedative containing a muscle relaxant with analgesic properties, xylazine was never intended for human consumption and therefore identified as a non-classified drug by the Food and Drug Administration (FDA), requiring only a veterinarian’s prescription. In humans, xylazine acts on the central nervous system and may cause drowsiness, slowed breathing, decreased heart rate, lowered blood pressure, and even amnesia. So when did this veterinary drug enter into the street drug supply chain, and how did it get there?