Fentanyl addiction remains a grave problem in the U.S., but another drug is creeping into the picture that could complicate the existing opioid crisis. Its street name is known as "tranq," short for tranquilizer, but its official name as a drug is xylazine. "It's been around since the 1960s, but mostly it was never really used in humans because of side effects," said Dr. Scott Phillips, who has a background in medical toxicology and serves as executive medical director at the Washington Poison Center. Xylazine is a sedative that is traditionally used as a tranquilizer for large animals like cows and horses and is not designed for humans.