Meat originating from U.S. horses may contain residues from substances banned by the U.S. Food and Drug Administration and the European Union for use in animals intended for consumption. Phenylbutazone, for example, is commonly administered to U.S. horses and has been associated with life-threatening reactions in humans. Requiring a thorough drug history for each U.S. horse intended for human consumption may help circumvent human health risks.
The Humane Society of the United States, "An HSUS Report: Food Safety Risks Associated With U.S. Horse Slaughter" (2012). EHH. 4.