The Drug Enforcement Agency (DEA) reports that it has seized xylazine and fentanyl mixtures in 48 of 50 states in the US. They mention that while fentanyl is responsive to naloxone, xylazine is not an opioid. Consequently, an overdose of a fentanyl/xylazine mixture may not respond to naloxone as expected.

DEA says that the great majority of fentanyl in the US comes from the Sinaloa and Jalisco cartels in Mexico, with the raw materials coming from China.

Xylazine is used in veterinary medicine as a sedative with analgesic and muscle relaxant properties. It would be interesting to see what the sulfur brings to the table as far as biological activity goes. You have to go to a bit of trouble to put the sulfur there so it must do something.

One reaction scheme for the preparation of xylazine is shown below. It is from expired US patent 4614798A (1985), Example 1, assigned to Vetamix. This prep uses acetic anhydride which is a List II chemical in the US. An acetamide intermediate is prepared from the aniline which is then reduced with sodium hydride and treated with carbon disulfide to give the reactive isothiocyanate intermediate. Finally, 3-aminopropanol adds to the isothiocyanate carbon to give the thiourea alcohol. Acid catalyzed cyclization by displacement of water by sulfur gives the xylazine product. This is some good meat-and-potatoes heterocyclic chemistry.

List II chemicals are subject to few serious reporting requirements in the US as seen in the table below. I suspect that the DEA benefits from good will disclosures. Following acetic anhydride shipment would be a good start in finding illicit labs in the US for several illegal drugs including xylazine, but it seems to be made in Mexico. Raw materials from China and drugs from Mexican labs add up to a very hard problem. Hard to tell how the cartel boys are making xylazine just from the internets.