Types of Heartworm Preventatives for Dogs and Administration Guide

HeartPreventionMeongsiljang Veterinary Advisory Board

There are three types of heartworm preventatives for dogs: oral, topical, and injectable. We’ve summarized the features, dosing schedules, and precautions for each based on veterinary evidence.

Why Is Heartworm Prevention So Important?

Illustration of a puppy protected by a heartworm prevention shield

Heartworm disease is a parasitic condition transmitted through mosquito bites. Once infected, adult worms reside in the heart and pulmonary arteries, potentially causing heart failure, lung damage, and even death. However, prevention is remarkably simple. Administering medication once a month, either orally or topically, provides highly effective protection. Veterinary textbooks emphasize that year-round preventive treatment is crucial for effectively safeguarding both dogs and cats. Preventive medications work by eliminating larvae before they can mature into adult worms after entering the body via mosquito bites.

Be Sure to Get an Infection Test Before Starting Preventive Medication

Administering preventive medication to an already infected dog can lead to serious side effects. Veterinary textbooks recommend confirming that the dog is healthy before starting preventive medication. If you are starting preventive medication for the first time or there has been a gap in administration, be sure to first check for infection through a blood test.

Oral — Medication to Feed

Ivermectin: This is the oldest active ingredient used for heartworm prevention. It is administered orally once a month and effectively kills larvae. However, because dogs of Collie-type breeds may have a genetic sensitivity to this drug, it is essential to consult with your veterinarian before use.

Milbemycin: This ingredient not only prevents heartworm but also simultaneously treats intestinal parasites such as roundworms and hookworms. According to veterinary pharmacology textbooks, the key to its effectiveness is administering the medication consistently on the scheduled date without missing any doses.

Oral heartworm medication and puppy illustration

Topical (Spot-on)

Selamectin: This topical treatment is applied to the back of the neck and helps prevent heartworm while simultaneously managing fleas and ear mites. Since age restrictions and safety during pregnancy or lactation may vary by product, always consult your veterinarian before use.

Moxidectin: Available in combination with imidacloprid, this all-in-one formula protects against heartworm, intestinal worms, and external parasites in a single application. It’s especially convenient for dogs that are difficult to give oral medication to.

Injectable — Medication Administered at the Veterinary Clinic

Moxidectin sustained-release injections can provide months of protection with a single dose. The exact duration of efficacy may vary depending on the specific product and your dog’s individual health status, so please consult your veterinarian before administration to confirm the appropriate schedule. This medication is only available for administration at veterinary clinics. The biggest advantage is that you don’t have to worry about missing doses. It’s a great option for pet owners who find it difficult to administer monthly medications.

Comparison of Preventive Medication Types

Type	Key Ingredients	Dosing Frequency	Additional Benefits	Special Notes
Oral	Ivermectin, Milbemycin	Once a month	Treats intestinal parasites	Use with caution in Collie breeds
Spot-on	Selamectin, Moxidectin	Once a month	Fleas and ticks	Ideal for dogs that are difficult to medicate orally
Injectable	Moxidectin sustained-release	Every few months (consult your veterinarian)	-	Administered at the clinic to prevent missed doses

Proper Administration Method

Administer medication on the same date each month: If more than 30 days pass between doses, larvae can develop into adult worms. Use a calendar or reminder app to keep track of your pet’s medication schedule.

Year-round administration is recommended: Veterinary textbooks strongly recommend year-round heartworm prevention. Skipping doses at any time of year can increase the risk of infection, so consistent administration is essential.

Use the correct dosage for your pet’s weight: Preventive medications are dosed based on weight. If your pet’s weight has changed, consult your veterinarian.

If your pet is already infected, the treatment process differs. Learn more about heartworm infection symptoms and treatment options.

Medication schedule calendar and dog illustration

Collie-type Breeds Must Consult with a Veterinarian

Breeds such as Collies, Shetland Sheepdogs, and Australian Shepherds may carry a mutation in the MDR1/ABCB1 gene. In dogs with this mutation, reduced or absent P-glycoprotein in the blood-brain barrier can allow high doses of macrocyclic lactone drugs, such as ivermectin, to penetrate the central nervous system, leading to neurotoxicity. The frequency of this allele has been reported to be approximately 70% in Collies. However, according to the veterinary pharmacology textbook Plumb's, the dose of ivermectin used for heartworm prevention (approximately 6 µg/kg) is considered safe even in dogs with the MDR1 mutation. Neurotoxicity becomes a concern only when doses significantly higher than those used for prevention are administered, such as for treating mange. Milbemycin and selamectin are also macrocyclic lactones but are generally safe at preventive doses. For added peace of mind, it is best to consult your veterinarian to prescribe the appropriate active ingredient and dosage tailored to your dog's breed and genotype.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

What should I do if I give my pet’s heartworm preventive medication a day late?

A delay of one or two days is generally not a major concern. Simply administer the medication as soon as you remember. However, if more than two weeks have passed, please contact your veterinarian and have your pet tested for infections if necessary.

Do indoor dogs also need heartworm prevention?

Yes, mosquitoes can enter through front doors or windows, so infections can occur indoors as well. For the safest protection, administer the medication once a month, regardless of whether your pet spends time indoors or outdoors.

Which is better, oral medication or topical medication?

The preventive effects are similar. If your dog refuses to take oral medication, a topical treatment may be more convenient, and if you also want to manage fleas, a spot-on product is a good choice. Please consult with your veterinarian to select the most suitable option.

Should I give my dog another dose if it vomits after taking medication?

If your pet vomits within one hour of taking the medication, it may not have been absorbed. Contact your veterinarian to determine whether a repeat dose is necessary. Do not increase the dose on your own.

Do cats take the same heartworm medication as dogs?

No, cats should not use medications intended for dogs. There are specific preventive medications formulated for cats, so please consult your veterinarian to get a prescription for a cat-specific product.