Yes, Heartgard is prescription-only in the United States. The FDA classifies it as a prescription drug, meaning you need a veterinarian’s authorization before you can buy it. In fact, every FDA-approved heartworm preventative on the market requires a prescription. There are no over-the-counter options.
Why Heartgard Requires a Prescription
The FDA restricts Heartgard to use “by or on the order of a licensed veterinarian” because safe use requires knowledge of veterinary parasitology. Specifically, a vet needs to assess your dog’s health before dispensing the medication, monitor for adverse reactions, and confirm your dog isn’t already infected with heartworms.
That last point is the most practical reason for the prescription requirement. Giving a heartworm preventative to a dog that already has an active heartworm infection can cause complications. Your vet needs to rule that out first, which means a blood test before you ever get a prescription.
The Testing That Comes Before a Prescription
The American Heartworm Society recommends that any dog over 7 months of age be tested for heartworms before starting a preventative. This involves two types of blood tests: one that detects heartworm proteins (antigens) and one that looks for microscopic heartworm larvae (microfilariae) circulating in the blood.
Puppies can start heartworm prevention as early as 8 weeks old without prior testing, since they haven’t had enough exposure time for an infection to develop. But those puppies still need to be tested 6 months after their first dose and then annually from that point forward.
If you’ve missed doses, switched brands, or don’t have testing records from the past 12 months, your vet will need to retest before writing or renewing a prescription. Annual screening is recommended for all dogs, even those who’ve been on preventatives consistently, because no medication is 100% effective and a missed or late dose can leave a window for infection.
What Heartgard Actually Contains
Heartgard Plus contains two active ingredients. The first, ivermectin, kills heartworm larvae that your dog may have picked up from mosquito bites in the previous 30 days. The second, pyrantel, treats common intestinal parasites like roundworms and hookworms. Each monthly chewable tablet handles both jobs at once.
The product comes in three dosage strengths based on your dog’s weight: up to 25 pounds, 26 to 50 pounds, and 51 to 100 pounds. Getting the right dose for your dog’s size is another reason the prescription step matters.
Breed Sensitivity to Ivermectin
Some dogs carry a genetic mutation called MDR1 that makes them highly sensitive to certain drugs, including ivermectin at higher doses. This mutation is especially common in herding breeds. About 70% of Collies carry it, along with roughly 50% of Australian Shepherds, 30% of McNabs and Silken Windhounds, and 15% of Shetland Sheepdogs and English Shepherds. German Shepherds, Border Collies, Old English Sheepdogs, and mixed-breed dogs can also be affected, though at lower rates.
The ivermectin dose in Heartgard is extremely low compared to the doses used for other purposes, and side effects at this dose level are generally not a concern even in sensitive breeds. Still, this is one more reason the prescription requirement exists: your vet can evaluate breed-related risks and recommend genetic testing if appropriate before starting any medication.
Can You Buy It Online Without a Prescription?
The FDA specifically warns consumers to be cautious of websites or stores selling heartworm preventatives without requiring a prescription. If a site lets you buy Heartgard (or any heartworm preventative) without a vet’s authorization, it’s either operating outside FDA regulations or selling unapproved products.
This isn’t just a U.S. rule. Canada also requires prescriptions for all heartworm prevention medications. Some countries, like Australia, sell certain heartworm products over the counter, but those products aren’t FDA-approved and can’t legally be imported for use in the U.S. or Canada. Online pharmacies sometimes exploit these cross-border regulatory differences to sell products without proper oversight.
Legitimate online pet pharmacies do exist and can fill Heartgard prescriptions, but they’ll contact your vet to verify the prescription before shipping. If a site skips that step entirely, that’s a red flag.
How to Get a Prescription
The process is straightforward. Schedule a vet visit, have your dog tested for heartworms, and if the test comes back negative, your vet will write a prescription. Many clinics sell Heartgard directly, but you’re also free to take that prescription to an online pharmacy or retail pet pharmacy if you find a better price. Vets are legally required to provide a written prescription if you ask for one.
Once you have an active prescription, refills typically only require an annual vet visit to retest and confirm your dog is still heartworm-free. Most vets will prescribe a full 12-month supply at a time.