What do you mean by supplement? Heartworm medication requires a prescription from your vet, but I think the Porter location has a Pet Vet. Link: https://www.tractorsupply.com/policies-information_pet-vet-clinic_pet-vet-clinic
Zack is correct. The dog has to have a negative HW test to prescribe preventive. Giving prevention to a positive dog can cause serious problems.
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