: Pets must be tested for existing heartworms before starting medication. Giving preventative meds to a pet that is already heartworm-positive can cause a dangerous, sometimes fatal, reaction.
: Major retailers with pet divisions, such as Walmart PetRx or Costco , often carry heartworm preventatives at lower prices. where to buy heartworm medication
You cannot buy genuine heartworm preventatives over the counter for several safety reasons: : Pets must be tested for existing heartworms