How Often Should Heartworm Pills Be Given to Dogs?

By Cheyenne Cartwright

Veterinarians prescribe heartworm pills for their canine patients because untreated heartworm disease is extremely debilitating and often fatal. To ensure your dog doesn't contract heartworms, you should administer the prescribed dosage on the same day each month.


Infected mosquitoes pass heartworms to the animals they bite. After a period of development as larvae in a dog's body, adult heartworms travel to and infest the animal's heart and blood vessels. They block the chambers of the heart and the arteries and veins, which can cause death.

Heartworm Preventatives

The most commonly prescribed heartworm pills for dogs are Heartgard, Heartgard Plus and Interceptor. Both Heartgard Plus and Interceptor also protect dogs from roundworms and hookworms. Tri-Heart Plus is a generic, less-expensive heartworm preventative that, according to Doctors Foster and Smith, has the exact formulation as Heartgard and is just as effective at treating heartworms.

Dosing Schedule

Dogs should be given heartworm pills on the same day of each month. The first dose will kill any existing heartworm infestations and protect your pet for the next month. Most vets recommend that you give your dog heartworm pills year-round. If not, you should dose your dog about a month after you see the first mosquito in your area.

Missed Doses

If you miss a dose, give your dog a heartworm pill as soon as you remember. If you miss dosing your dog for three months or more, contact your vet to get your dog tested for heartworms before you resume dosing.

Heartworm Testing

Drs. Foster and Smith recommend that you have your dog tested every year for heartworms. They also suggest that your vet should test your dog before you change his medication. Because it takes at least six and a half months for heartworm larvae to develop into adult worms, puppies younger than seven months can start taking preventatives without being tested.