Selamectin for Dogs: Dosage, Side Effects, and How to Use It
By Dr. Zubair Khalid, DVM, MS, PhD ·
Selamectin for dogs dosage is calculated by your veterinarian from your dog's exact body weight, then applied to the skin once a month. It is a prescription-only medication in the United States, so you cannot buy it legally without a veterinarian's approval. Most dogs tolerate it well, but side effects can happen and some dogs should not receive it at all.
This article is educational and is not a substitute for veterinary diagnosis or treatment.
Quick Answer
- Selamectin is a topical monthly parasite preventive that kills fleas, some ticks, ear mites, sarcoptic mange mites, and prevents heartworm disease in dogs.
- The dose is based on body weight and is measured in milligrams per kilogram (mg/kg). Your veterinarian calculates it, and you apply the pre-filled tube that matches your dog's weight range.
- It is a prescription product in the U.S. You need a valid veterinarian-client-patient relationship, which usually means a recent exam and a negative heartworm test.
- Common side effects are mild and temporary, such as hair loss or itching at the application site. Serious neurologic signs are rare but possible, especially in dogs with a specific genetic sensitivity [1].
- If you see wobbling, tremors, dilated pupils, drooling, or weakness after application, call your veterinarian and Pet Poison Helpline at (855) 764-7661 right away [1].
What Selamectin Is and What Veterinarians Use It For
Selamectin belongs to a drug class called macrocyclic lactones. These medicines are widely used in veterinary parasitology to control both internal and external parasites [2][3]. In dogs, selamectin is applied to the skin and absorbed into the bloodstream, where it works against several types of parasites at once.
Veterinarians commonly prescribe selamectin for:
- Flea control and flea allergy dermatitis
- Heartworm prevention
- Ear mite treatment
- Sarcoptic mange (scabies)
- Some tick species, depending on the product formulation
Selamectin is also combined with other drugs in some products to broaden the range of parasites covered. For example, a combination of selamectin plus sarolaner is used to kill ticks before they can transmit certain infections [4]. Your veterinarian will choose the specific product based on your dog's risk factors, lifestyle, and region.
How Selamectin Works
After you apply selamectin to your dog's skin, it is absorbed through the skin and into the bloodstream. From there it distributes to tissues and reaches the parasites that feed on your dog. It works by interfering with the parasite's nervous system, which paralyzes and kills susceptible parasites.
Because it enters the bloodstream, selamectin can reach heartworm larvae that mosquitoes have deposited in your dog's body. This is why it works as a heartworm preventive. It also reaches fleas and mites on and in the skin.
Macrocyclic lactones like selamectin are generally effective, but resistance is an emerging concern in canine parasitology. Reports of macrocyclic lactone resistance in heartworm and multidrug resistance in hookworms are increasing worldwide [3]. This is one reason year-round prevention and regular veterinary testing matter.
Selamectin for Dogs Dosage: What Owners Need to Know
Who Decides the Dose
Your veterinarian decides the dose. Selamectin is not dosed by guesswork or by eyeballing your dog's size. The dose is calculated in milligrams per kilogram of body weight, and the product comes in pre-filled tubes designed for specific weight ranges. If your dog is between weight ranges, your veterinarian will tell you which tube to use.
Never split, combine, or reuse tubes to try to match a dose yourself. The concentration of the medication in each tube is fixed, and using the wrong tube can underdose or overdose your dog.
How the Dose Is Given
Selamectin for dogs is topical. You squeeze the entire contents of one tube onto the skin at the base of the neck, between the shoulder blades. This spot is chosen because your dog cannot easily lick it off. The medication is absorbed through the skin over several hours.
Do not massage the product into the skin unless your veterinarian tells you to. Do not bathe your dog or let them swim for at least a few hours after application, because water can wash the medication away before it is absorbed.
Selamectin for Dogs Dosage Chart
A true dosage chart depends on the exact product, its concentration, and your dog's weight. Because selamectin is prescription-only and comes in weight-banded tubes, the safest chart is the one your veterinarian gives you for the specific product you are using.
| Dog's weight range | What your veterinarian will do |
|---|---|
| Under 5 lbs | Select the smallest available tube and confirm it is labeled for your dog's weight |
| 5 to 10 lbs | Match the tube to this weight band |
| 10 to 20 lbs | Match the tube to this weight band |
| 20 to 40 lbs | Match the tube to this weight band |
| 40 to 80 lbs | Match the tube to this weight band |
| Over 80 lbs | May require a combination of tubes, decided by your veterinarian |
The numbers above are weight bands, not doses. The actual milligram amount depends on the product. Your veterinarian will confirm the correct tube for your dog.
What to Do About a Missed Dose
If you miss a monthly dose, apply it as soon as you remember. Then restart your monthly schedule from that new date. If more than a month has passed, your dog may have a gap in heartworm protection. Call your veterinarian to ask whether a heartworm test is needed before you restart. Do not double up on doses to make up for a missed one.
Selamectin for Dogs Side Effects
Most dogs have no reaction at all. When side effects do occur, they are usually mild and resolve on their own. The table below lists what owners most often see and what to do.
| Side effect | How common or serious | What to do |
|---|---|---|
| Hair loss at the application site | Uncommon, mild | Note it and mention it at your next visit. Do not reapply. |
| Itching, redness, or irritation at the site | Uncommon, mild | Keep your dog from licking the area. Call your vet if it spreads. |
| Drooling or brief nausea | Uncommon, mild | Watch closely. Call your vet if it continues. |
| Lethargy or decreased appetite | Uncommon, can be serious | Call your veterinarian the same day. |
| Wobbling, weakness, or ataxia | Rare, serious | Call your veterinarian and Pet Poison Helpline at (855) 764-7661 immediately [1]. |
| Dilated pupils or tongue protrusion | Rare, serious | This can signal neurologic toxicity. Seek emergency care now [1][5]. |
| Tremors or seizures | Rare, emergency | Go to an emergency clinic immediately. |
Neurologic signs after macrocyclic lactone exposure are a known risk in animals with a genetic mutation that affects a drug transporter called P-glycoprotein [5]. Dogs with this sensitivity can react to doses that are safe for other dogs. This is why your veterinarian asks about breed, prior reactions, and any history of neurologic signs before prescribing.
If your dog has ever reacted to a parasite preventive, tell your veterinarian before starting selamectin. If you are managing other medications at home, our guide to Benadryl (diphenhydramine) dosage for dogs explains how weight-based dosing works for common over-the-counter drugs, and our article on Vetmedin side effects in dogs covers what to watch for with heart medications.
Who Should Not Take Selamectin and Drug Interactions
Selamectin is not right for every dog. Your veterinarian may avoid it or choose a different product if your dog:
- Has a known allergy to selamectin or similar macrocyclic lactones
- Has a history of neurologic reactions to parasite preventives
- Is a breed or line known to carry the genetic mutation that causes macrocyclic lactone sensitivity [1][5]
- Is underweight, debilitated, or very young, unless your veterinarian specifically approves it
- Is currently taking other medications that affect the same drug transporter pathways
Drug interactions matter. Macrocyclic lactones share pathways with other drugs, and animals with a non-functional P-glycoprotein have an increased risk of neurotoxicity from macrocyclic lactones and other P-glycoprotein substrate drugs [5]. Tell your veterinarian about every medication, supplement, and topical product your dog receives, including flea products from other brands. Using two products that contain the same or similar drugs at the same time can raise the risk of toxicity.
If your dog takes medication for anxiety or allergies, review our article on melatonin for dog anxiety and our guide to Benadryl for dogs by weight before adding anything new to their routine.
Overdose and When to Call a Veterinarian or Poison Control
An overdose can happen if your dog licks off the medication, if you apply too much, or if your dog eats the tube. Signs of macrocyclic lactone toxicity include tongue protrusion, dilated pupils, generalized weakness, and ataxia [5]. These signs can appear after a routine application in a sensitive animal, so do not assume a normal dose is always safe [5].
If you suspect an overdose or see neurologic signs, contact your veterinarian and Pet Poison Helpline at (855) 764-7661 immediately [1]. Without prompt and aggressive treatment, ivermectin poisoning can be fatal, and the same principle applies to related macrocyclic lactones [1]. Treatment may include inducing vomiting and giving activated charcoal to bind the toxin, but only a veterinarian should decide that [1]. Do not try to make your dog vomit at home unless a veterinarian tells you to.
Limitations and When to Contact a Veterinarian
Selamectin is a preventive, not a treatment for an active heartworm infection. Giving it to a heartworm-positive dog without veterinary guidance can be dangerous. Your veterinarian will test before prescribing and will set up a treatment plan if your dog is positive.
Contact your veterinarian the same day if you see lethargy, appetite loss, vomiting, or irritation at the application site that is spreading. Go to an emergency clinic now if you see wobbling, weakness, dilated pupils, tongue protrusion, tremors, or seizures [1][5]. Schedule a routine visit if you are unsure which product to use, if your dog's weight has changed, or if you are traveling to an area with different parasite risks.
Every dog is different. Your veterinarian needs to examine your dog and review their history before deciding whether selamectin is safe and appropriate.
Frequently Asked Questions
Do I need a prescription for selamectin for dogs?
Yes. Selamectin is a prescription medication in the United States, so you cannot legally buy it without a veterinarian's authorization. A valid veterinarian-client-patient relationship is required, which usually means a recent exam and a current negative heartworm test. Buying prescription parasite preventives from unverified online sellers puts your dog at risk of receiving a counterfeit or wrong-strength product.
How much does selamectin for dogs cost?
Prices vary by product, size of tube, and where you buy it, so there is no single number that applies to every dog. Your veterinarian's office, a veterinary pharmacy, and online pet pharmacies may all charge different amounts. Ask your veterinarian for a written estimate and compare it with a licensed pharmacy before you decide.
Can I use selamectin on a puppy?
Only if your veterinarian approves it for your puppy's age and weight. Very young or underweight puppies may not be good candidates, and the decision depends on the specific product label and your puppy's health. Never use a product labeled for adult dogs on a puppy without veterinary guidance.
What happens if my dog licks selamectin off after application?
Mild drooling or brief nausea can happen if your dog licks the application site. Apply the product between the shoulder blades where your dog cannot reach, and keep them from grooming that area for a few hours. If your dog shows wobbling, weakness, or other neurologic signs, call your veterinarian and Pet Poison Helpline at (855) 764-7661 right away [1].
Can selamectin be used with other flea and tick products?
Sometimes, but only with your veterinarian's approval. Using two products that contain similar drugs can increase the risk of toxicity, especially in dogs with macrocyclic lactone sensitivity [5]. Tell your veterinarian about every product your dog is already using, including shampoos, sprays, and chewables, before you add selamectin.
Related Articles
- Benadryl (Diphenhydramine) Dosage Calculator for Dogs: Veterinary Dosage Guide
- What Are the Side Effects of Vetmedin for Dogs?
- Can You Give Your Dog Melatonin for Anxiety? Safety and Dosage
- Benadryl Diphenhydramine for Dogs: Weight-Based Dosing for Allergies
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References
- Understanding the Symptoms and. Pet Poison Helpline.
- A retrospective comparison of two-dose and three-dose melarsomine, with doxycycline and a macrocyclic lactone, in 1246 shelter dogs. Parasites & vectors, 2026.
- Anthelmintic resistance in canine parasitology: a critical review of an emerging global challenge. Animal health research reviews, 2026.
- Efficacy of Revolution® Plus (selamectin plus sarolaner) for the prevention of transmission of Borrelia burgdorferi from infected Ixodes scapularis to cats. Parasites & vectors, 2026.
- Inherent P-glycoprotein deficiency and macrocyclic lactone toxicity in an Australian Maine Coon cat homozygous for ABCB11930_1931del TC. JFMS open reports, 2026.