How HeartGard for dogs works:
When given monthly as directed, HEARTGARD Plus is effective in helping to prevent heartworm disease in dogs. Each bite from an infected mosquito can introduce new heartworm larvae into your dog's bloodstream and reinfect an unprotected dog. Use of HEARTGARD Plus every month as directed kills immature heartworms introduced to the dog in the last 30 days, preventing the development of adult heartworms.
HEARTGARD Plus also kills canine roundworms and hookworms in the dog's intestinal tract. Learn more.