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 & 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.
Concerns have been raised on the impact on zilpaterol on flavor, and the FDA states that overall tenderness, juiciness, flavor intensity, and beef flavor were all statistically different in Zilmax-treated beef compared to controls. However, several studies have shown the use of ziplaterol leads to increased size, feed efficiency, and value.
Merck reported Zilmax-fed cattle do not produce beef with a difference in taste or quality compared to cattle not fed the drug, but elsewhere, concerns have been raised about the beef's tenderness. Studies have variously found a slight reduction in tenderness, an increase in shear force, and a lower percentage of intramuscular fat (marbling).