Preventive medications come in several forms, including chewable pills, topical “spot-ons” and injectable medications. Because one form or another may be easier for owners to give their dogs and because some—but not all—products may prevent multiple parasites besides heartworms, owners and veterinarians should work together to determine the best product for an individual pet. The American Heartworm Society recommends year-round administration of heartworm preventives to ensure pets are protected from deadly heartworms.