In 1981, Merck began commercialize ivermectin as a broad-spectrum veterinary anti-parasitic drug that was originally developed to treat heartworm and acariasis in animals. The mixture was 25 times more potent than existing treatments for parasitic worms.

Starting in 1987, the drug was marketed to the public under the brand name Heartgard^® (now sold by the animal-health company Merial) to prevent heartworms in dogs. These products became the top-selling veterinary medicines in the world, with sales topping $1 billion annually.