In West Virginia and elsewhere, dealers mix fentanyl with the powerful animal sedative xylazine. NBC News was able to arrange overseas purchases of the drug within minutes.

Dr. Steven Corder didn’t think his job treating people addicted to fentanyl in Wheeling, West Virginia, could get any harder, but then he began encountering patients who were addicted to both fentanyl and a second drug with its own destructive power — the livestock tranquilizer xylazine.

“Opioid withdrawal is hard enough,” Corder said. But his usual tools, he lamented, “couldn’t touch the withdrawal from xylazine.”

Xylazine is now present in one out of every nine overdose deaths nationwide involving illicit fentanyl, according to the Centers for Disease Control and Prevention.