Xylazine was first created in 1962 by the German pharmaceutical company Farbenfabriken Bayer as a potential blood pressure medication for humans. It was never approved for people, but by the late 1960s it found a role as a sedative and pain reliever for animals. Today, the xylazine showing up in the illicit drug supply comes primarily from bulk chemical shipments out of China and, to a lesser extent, from diversion of veterinary products within the United States.
How a Veterinary Drug Was Born
Bayer originally developed xylazine as an analog of clonidine, a drug still used in humans to lower blood pressure and treat withdrawal symptoms. The two compounds share a similar chemical backbone and work on the same type of receptor in the nervous system, called an alpha-2 receptor. Activating that receptor slows heart rate, lowers blood pressure, and produces heavy sedation.
In animal testing, xylazine turned out to be too sedating for human use but ideal for veterinary medicine. It became widely used to sedate horses, cattle, and other large animals before procedures. The FDA approved it strictly for veterinary purposes, and it remains legal to buy and sell for that use today. Critically, xylazine is not classified as a controlled substance under federal law, though some states have added it to their own controlled substance lists. That gap in regulation is a major reason it has become so easy to obtain in bulk.
Where the Illicit Supply Originates
A joint investigation by the DEA and the Department of Homeland Security identified several routes xylazine enters the U.S. for illicit use. The largest source is bulk powder shipped from chemical suppliers in China and other countries. A kilogram of xylazine powder can be purchased online from Chinese suppliers for as little as $6 to $20, making it extraordinarily cheap to acquire in large quantities. For comparison, that same kilogram can be used to cut and stretch far more expensive drugs like fentanyl, creating a strong financial incentive for dealers.
Xylazine also enters the supply in liquid form, either diverted from legitimate veterinary channels or repackaged to look like a veterinary product. Online sellers market liquid and powder xylazine openly, often with no requirement that buyers prove any connection to animal care. A smaller share is intercepted already mixed with fentanyl at the southwest border.
Puerto Rico: The Early Warning
Long before xylazine made national headlines on the U.S. mainland, it was already a fixture in Puerto Rico’s drug market. Injecting drug users on the island reported using xylazine as early as the 2000s, where it earned the street name “Anestesia de Caballo,” or Horse Anesthetic. Dealers initially sold it alongside heroin, letting buyers control the ratio themselves. Over time, pre-mixed products appeared, some containing as much as 80% xylazine and only 20% heroin. One popular combination called “el combito” (the small combo) bundled cocaine, heroin, and xylazine together in a single package for a few dollars.
Puerto Rico’s experience foreshadowed what would happen on the mainland: once xylazine proved cheap and easy to source, it spread rapidly through drug markets that were already built around heroin and, later, fentanyl.
How It Ends Up Mixed With Fentanyl
Xylazine is not typically sold on its own at the street level. It is almost always mixed into other drugs, most commonly fentanyl. Dealers add it because it is cheap, unregulated, and extends the sedative effect of fentanyl, which on its own wears off relatively quickly. Users may not know xylazine is present in what they bought.
Pharmaceutical xylazine comes as a liquid, and converting it into a form that blends evenly into heroin or fentanyl powder requires some skill. The availability of cheap bulk powder from overseas has simplified this process considerably, since powder mixes more easily with other powders. The result is that xylazine contamination has climbed steeply in fentanyl supplies across the country. In Los Angeles, for example, xylazine went from being undetectable in fentanyl samples in early 2023 to showing up in roughly 1 out of every 4 to 5 fentanyl samples by early 2025.
Why It’s Hard to Control
The core problem is regulatory. Because xylazine is not a federally controlled substance, buying, selling, and shipping it carries none of the legal restrictions that apply to opioids or other scheduled drugs. Veterinarians can purchase it through standard pharmaceutical distributors. Anyone else can find it on general chemical supply websites with minimal scrutiny. Chinese chemical manufacturers sell it in bulk with no questions asked, and international shipments of white powder are difficult to intercept at scale.
Some states have responded by scheduling xylazine independently, which gives law enforcement more tools to pursue cases involving the drug within their borders. But as long as the compound remains cheap, legal at the federal level, and globally available online, supply-side enforcement faces an uphill battle. The same qualities that made xylazine useful in veterinary clinics for decades, its potent sedation at low cost, are exactly what make it attractive to people looking to stretch the illicit fentanyl supply.