A case report of the treatment of an overdose of xylazine in a cow.

An adult Holstein cow was inadvertently administered an overdose of xylazine hydrochloride. The cow became weak, collapsed, and developed severe bradycardia and intermittent heart block. Initial therapy for cardiovascular collapse consisted of rapid infusion of large volumes of intravenous fluids and intravenous injection of atropine sulfate. Yohimbine, an alpha 2-adrenergic antagonist, and tolazoline hydrochloride, an alpha 1- and alpha 2-adrenergic antagonist, were administered intravenously as antidotes for xylazine overdose. The cow recovered but calved prematurely 4 days later.