Ivermectin and Selamectin for Dog Heartworm Prevention

Name: Ivermectin and Selamectin for Dog Heartworm Prevention
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

Ivermectin for dogs heartworm prevention is a monthly medication that kills the immature larval stages of heartworms before they can grow into adult worms in your dog's heart and lungs. Selamectin does the same job and is often sold as a topical product that also covers fleas and some ticks. Both are prescription preventives, and neither one treats an existing adult heartworm infection.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

Both ivermectin and selamectin are macrocyclic lactones, the only drug class currently licensed to prevent heartworm disease in dogs [1].
These drugs kill third- and fourth-stage heartworm larvae, which stops adult worms from developing [1].
Ivermectin is typically given by mouth once a month, while selamectin is usually applied to the skin once a month. Your veterinarian decides the exact dose based on your dog's weight and health.
A genetic mutation makes some dogs (often described as "white feet, do not treat") more sensitive to ivermectin, so your veterinarian needs your dog's breed history before prescribing [2].
Heartworm prevention does not kill adult heartworms, so a dog that tests positive needs a different treatment plan [3].

What Ivermectin and Selamectin Are and What Veterinarians Use Them For

Ivermectin and selamectin belong to a drug class called macrocyclic lactones. This is the only drug class currently licensed for heartworm disease prophylaxis [1]. That means when your veterinarian prescribes a heartworm preventive, it almost always contains one of these compounds or a close relative like moxidectin or milbemycin.

Heartworm disease is caused by parasitic worms that live in the arteries of the lungs and the right side of the heart [4]. The disease has been reported in all 50 states, though it is more common in some regions than others [4]. Mosquitoes spread it. When an infected mosquito bites your dog, it deposits immature larval forms called microfilariae into the bloodstream [3].

Veterinarians use these drugs for one main purpose: prevention. They are not used to kill adult heartworms. If your dog already has adult worms, your veterinarian will recommend a separate treatment protocol. You can read more about that distinction in Heartworm Disease in Dogs: Life Cycle Prevention and Melarsomine Treatment.

Selamectin is often chosen when owners want one product that also handles fleas and some ticks. Ivermectin is often chosen for its low cost and long track record. Both are effective when given correctly and on schedule.

How These Drugs Work Against Heartworm

Heartworm prevention works by killing the microfilariae that mosquitoes inject [3]. If those larvae are not killed, they travel to the heart and mature into adult worms over a period of months [3]. Female adult heartworms can reach 10 to 12 inches in length and live up to 7 years [3].

The timing matters. Macrocyclic lactones kill third- and fourth-stage larvae of Dirofilaria immitis, which prevents adult worms from developing [1]. This is why monthly dosing is so important. The drug does not stay in the body long enough to cover a whole season. It clears the larvae that arrived in the past month, then it is gone.

Adult heartworms damage the heart muscle, and your pet's heartworm preventive cannot kill these adult heartworms [3]. That is the core reason prevention and treatment are different. Prevention is cheap, safe, and simple. Treatment carries much greater risk, difficulty, and cost [3].

Dosing Basics for Ivermectin and Selamectin

Who decides the dose

Your veterinarian does. The dose depends on your dog's weight, age, health status, and the specific product. Never split a product made for a large dog to give to a small dog, and never guess at a dose. Some products are dosed by weight bands, and getting it wrong can mean underdosing (which fails to prevent heartworm) or overdosing (which can cause toxicity).

How it is given

Ivermectin preventives are usually given by mouth as a chewable tablet or flavored tablet once a month. Selamectin is usually applied as a topical liquid between the shoulder blades once a month. Follow the label directions exactly. For topicals, part the hair and apply directly to the skin so your dog cannot lick it off. Keep dogs separated after application until the product dries.

What to do about a missed dose

Give the missed dose as soon as you remember, then resume your normal monthly schedule. If more than a month or two has passed, call your veterinarian. They may want to test your dog for heartworm before restarting, because a gap in coverage means your dog may have been exposed. Annual testing is part of good prevention practice, and current guidelines characterize prescribing these preventives without antigen testing as "NOT RECOMMENDED," regardless of your compliance history or where you live [5].

Why year-round dosing matters

Many owners stop preventives when the weather turns cool. That is a mistake. A late-summer or early-fall mosquito bite can still spell big trouble for your pet [3]. Year-round prevention is one of the most important, cost-effective investments you can make [4]. If you want the bigger picture on why consistency matters, see Heartworm Prevention for Dogs: Why It's Essential and How It Works.

Side Effects: Common and Serious

Most dogs tolerate ivermectin and selamectin well. Side effects are usually mild and short-lived. The table below covers what owners most often see.

Side effect	How common or serious	What to do
Vomiting, diarrhea, drooling, or loss of appetite	Common and usually mild	Withhold food for a few hours, offer water, and call your veterinarian if it lasts more than a day
Lethargy or mild sleepiness	Common and usually mild	Let your dog rest and monitor for 24 hours
Redness, itching, or hair loss at the topical application site	Common with selamectin	Do not let your dog lick the area, and call your veterinarian if it worsens
Tremors, wobbling, incoordination, or dilated pupils	Serious, especially in ivermectin-sensitive dogs	Call your veterinarian or poison control right away
Seizures, blindness, or coma	Serious and potentially life-threatening	This is an emergency. Go to a veterinary hospital now
Reduced tear production (dry eye)	Possible even at recommended doses	Ask your veterinarian about tear testing if your dog has a history of eye problems

Ivermectin can cause neurological damage and potential seizures in sensitive dogs [2]. Symptoms of ivermectin poisoning can begin within 4 to 12 hours after ingestion, while more gradual signs may take up to 2 or 3 days to appear [2]. A recent study found that routine systemic ivermectin treatment was associated with reduced aqueous tear production, and even at recommended doses it may predispose susceptible dogs to subclinical dry eye [6]. That is one more reason to keep regular veterinary visits on the calendar.

Who Should Not Take These Drugs and What Interacts With Them

The biggest caution with ivermectin is the MDR1 gene mutation. Certain dog breeds are born with a genetic mutation that makes them more sensitive to ivermectin [2]. The old rule of thumb is "white feet, do not treat" [2]. Breeds commonly associated with this sensitivity include collies, Australian shepherds, Shetland sheepdogs, and other herding breeds, along with some mixed breeds that carry the gene.

If your dog is one of these breeds, that does not automatically mean ivermectin is off the table. It means your veterinarian needs to know your dog's breed background and may choose a different preventive or a lower dose. Never use a livestock or over-the-counter ivermectin product on your dog. Those products are made for much larger animals and can easily cause toxicity in a dog.

Other situations where your veterinarian may avoid or adjust these drugs:

Puppies under the minimum age or weight listed on the product label
Dogs that are sick, debilitated, or recovering from surgery
Dogs taking other medications that affect the nervous system
Dogs with a known allergy to the active ingredient
Dogs that have not been tested for heartworm

Tell your veterinarian about every medication, supplement, and topical product your dog receives. Drug interactions can change how these preventives work or how your dog handles them.

Overdose and When to Call Poison Control

Ivermectin overdose is a real risk, especially in small dogs. A 3-year-old Shih Tzu weighing 2.5 kg developed ataxia, tremors, incoordination, loss of pupil reflexes, bradycardia, and salivation after being given a 10 mg ivermectin tablet [7]. That dog required hospitalization and intravenous lipid emulsion therapy to recover [7].

If you suspect your dog ate too much of a preventive, or ate a livestock ivermectin product, do not wait for symptoms. Call your veterinarian or one of these poison control lines right away:

ASPCA Animal Poison Control Center: (888) 426-4435
Pet Poison Helpline: (855) 764-7661

Have the product packaging ready so you can tell them the active ingredient, the concentration, and how much your dog may have swallowed. Do not induce vomiting unless a veterinarian tells you to.

Limitations and When to Contact a Veterinarian

These medications prevent heartworm. They do not treat it. If your dog already has adult heartworms, a preventive will not clear the infection, and giving one to a heartworm-positive dog can sometimes cause a reaction as microfilariae die off.

Call your veterinarian the same day if your dog vomits right after a dose, has diarrhea that lasts more than 24 hours, seems unusually tired, or has redness or swelling where a topical was applied.

Go to an emergency veterinary hospital now if your dog has tremors, wobbling, seizures, dilated pupils, trouble breathing, or collapses. These are signs of ivermectin toxicity and they can progress quickly [2][7].

Schedule a routine visit if your dog has missed one or more monthly doses, if you are switching products, or if it has been more than a year since the last heartworm test. Annual antigen testing is part of responsible prevention, and a risk-based approach that considers local prevalence and your compliance history is worth discussing with your veterinarian [5].

Every dog is different. Your veterinarian is the only person who can tell you which preventive fits your dog, at what dose, and on what schedule.

Frequently Asked Questions

Is ivermectin safe for all dogs?

No. Dogs with the MDR1 gene mutation are more sensitive to ivermectin and can develop neurological signs even at normal doses [2]. Your veterinarian can test for the mutation or choose a different preventive if your dog is in a high-risk breed group.

Can I use a livestock ivermectin product on my dog?

No. Livestock products are formulated for much larger animals and are a common cause of ivermectin poisoning in dogs. Only use a preventive that your veterinarian prescribes specifically for your dog.

Does my dog need a heartworm test before starting a preventive?

Yes, in most cases. Current guidelines characterize prescribing these preventives without antigen testing as "NOT RECOMMENDED," even for dogs with a good compliance history [5]. Testing protects your dog from a reaction and confirms the preventive is the right choice.

What happens if I miss a month of prevention?

Give the missed dose as soon as you remember and then return to your monthly schedule. If the gap was longer than a month or two, call your veterinarian, because they may want to retest your dog before restarting.

Can heartworm prevention kill adult heartworms in my dog?

No. These drugs kill the larval stages that mosquitoes inject, not the adult worms living in the heart and lungs [3][1]. A dog with adult heartworms needs a separate treatment plan prescribed by a veterinarian.

References

Efficacy and safety of a sustained-release formulation of ivermectin (FILAPREV®) in preventing heartworm infection (Dirofilaria immitis) in dogs in two endemic areas of Italy. Parasites & vectors, 2026.
Understanding the Symptoms and. Pet Poison Helpline.
Heartworm Preventatives. Pet Poison Helpline.
Heartworm Prevention In Dogs. Pet Poison Helpline.
Is annual Dirofilaria immitis antigen testing at preventive renewal proportionate to the residual infection risk in documented-compliant dogs?. Journal of the American Veterinary Medical Association, 2026.
Systemic Ivermectin Administration Reduces Aqueous Tear Production and Affects Ocular Surface Health in Clinically Healthy Dogs. Veterinary ophthalmology, 2026.
Intralipid emulsion therapy for ivermectin toxicity in a dog. Veterinary medicine and science, 2025.