Just use Heartgard® (ivermectin) Chewables once a month and your cat will be protected from the threat of heartworm disease. In a tasty flavor that cats love, Heartgard Chewables is safe for all sizes and breeds of cats. It is approved for use in kittens as young as 6 weeks, small cats regardless of weight, pregnant or breeding queens, and toms. Heartgard Chewables for Cats is available only through prescription from a licensed veterinarian.