Heartworm infestation can kill your dog, but prescription heartworm preventive medication keeps him protected. Before your vet writes the prescription, she conducts an annual blood test on your pet to ensure your dog is heartworm-free. Even if your dog is on heartworm medication year-round, this test is crucial to his health. Giving your dog the medication if he's infected with heartworms can be deadly, so heartworm medication is available only by prescription.
Heartworm is spread to dogs via the bite from an infected mosquito. Heartworm medication works by killing off larvae before they get the opportunity to grow to adulthood. However, if you dog already has adult heartworms, the preventives are not useful as they're not designed to kill mature worms. Giving a heartworm medication to an infested dog can result in the baby worms circulating in his bloodstream to suddenly die off, which can cause the dog to go into shock and die.