In the United States, a prescription from a licensed veterinarian is legally required to purchase Heartgard Plus, as it is classified as a prescription-only medication. This regulation ensures safety, as administering prevention to a heartworm-positive dog can cause severe reactions, and it guarantees access to authentic, manufacturer-guaranteed medication. For more details on heartworm prevention requirements, visit American Heartworm Society . Why Heartworm Prevention Is Necessary | Vet Castle Rock, CO