Zilpaterol (zilpaterol hydrochloride) is a beta-adrenergic agonist. Under its trade name, Zilmax, it is used to increase the size of cattle and the efficiency of feeding them. Zilmax is produced by Intervet, a subsidiary of Merck and Co., and marketed as a “beef improvement technology”. Zilpaterol is typically fed in the last three to six weeks of cattle’s lives, with a brief period (three days in the US) before death for withdrawal, which allows the drug to mostly leave the animal’s tissues.