Harm reduction groups, health officials and law enforcement members are warning of the rise of Xylazine, or 'tranq', a powerful horse tranquilizer that is being mixed with street drugs. WATCH NIGHTLINE EPISODES: abc.go.com/shows/nightline ALSO AVAILABLE ON HULU: hulu.tv/2wSmSrZ #nightline #xylazine #tranq #opioids #fentanyl #overdose #drug #abcnews