Be careful buying heart worm medication online. If your dog should get heart worm while on medication (rare) the manufacturers won't pay for treatment if not bought from authorized vets. Dr..Fosters and Smith has their own guarantee.. I prefer to get my medications from my vet. I have purchased from Dr. Fosters and Smith when my vet was unable to get a medication due to a national shortage.
Drs. Foster & Smith Pharmacy guarantees all of the prescription heartworm preventives they sell