Selamectin is a broad-spectrum antiparasitic drug primarily used in dogs and cats to prevent and treat infestations from fleas, heartworms, ear mites, sarcoptic mange, and certain ticks. It belongs to the avermectin class of macrocyclic lactones, which are derived from naturally occurring compounds produced by Streptomyces bacteria. The development of selamectin builds on the discovery of avermectins in the late 1970s by William C. Campbell and Satoshi Ōmura, which led to the creation of ivermectin and other derivatives that transformed parasite control in both humans and animals. Selamectin was specifically designed for safe, topical use in companion animals, allowing convenient monthly administration while providing systemic protection. Marketed under the brand name Revolution® by Zoetis (formerly Pfizer Animal Health), it was first approved by the FDA in 2000. Its introduction marked a significant advancement in veterinary medicine, offering a safe, effective, and easy-to-use option for broad-spectrum parasite prevention and treatment.
BRAND NAMES
Primary brand names
Revolution: One of the most well-known and original brand names for selamectin. It is available for both dogs and cats.
Stronghold: The brand name used for selamectin in Europe and other international markets.
Generic and regional brand names
Revolt: A generic brand available in the United States.
Selamec: An Indian brand name for selamectin.
Selarid: A generic brand available in the United States.
Senergy: A generic brand available in the United States.
Chanhold: A brand name used in Europe.
Evicto: A brand name available in Canada.
Paradyne: A brand name available in the United States.
Selevectine: A brand name available in Canada.
MECHANISM OF ACTION
Selamectin works by binding to and activating glutamate-gated chloride channels in the nerve and muscle cells of parasites. This increases the flow of chloride ions, leading to hyperpolarization, paralysis, and ultimately death of the parasite. By disrupting neuromuscular transmission in invertebrates such as nematodes and arthropods, selamectin effectively eliminates infestations. Its safety in mammals is attributed to the blood-brain barrier, which prevents the drug from affecting the same receptors in the central nervous system.
PHARMACOKINETICS
Absorption
Selamectin is absorbed through the skin when applied topically and is distributed systemically, reaching peak plasma levels within 1 day in cats and 3 days in dogs. It is eliminated slowly, with a long half-life of approximately 8 days in cats and 11 days in dogs, which allows for convenient monthly dosing. Because oral absorption is poor, selamectin is specifically formulated for topical use.
Distribution
After topical administration, it distributes widely throughout the body, including skin, sebaceous glands, and fatty tissues, which helps maintain effective concentrations over time. This extensive distribution, along with its slow elimination, contributes to its long half-life (approximately 8 days in cats and 11 days in dogs) and supports monthly dosing for parasite control.
Metabolism
Selamectin is metabolized into a major metabolite that is detectable in the liver, gastrointestinal tract, and excreta. Although the precise metabolic pathway is not fully characterized, it is known that the drug is processed and eventually excreted after administration. Its metabolism may be influenced by other medications, which could potentially reduce the clearance of certain drugs.
Excretion
Selamectin is mainly excreted in the feces, with only a small portion appearing in the urine, primarily as the unchanged drug. In both dogs and cats, fecal elimination is the predominant route, although the drug is cleared more rapidly in dogs than in cats.
PHARMACODYNAMICS
Selamectin’s pharmacodynamic effect is to paralyze and kill invertebrate parasites by targeting their nerve and muscle cells. It functions as a GABA agonist and binds to glutamate-gated chloride channels, causing an influx of chloride ions. This disrupts normal neurotransmission, resulting in paralysis and death of parasites such as fleas, heartworms, and other nematodes and arthropods. Its high specificity for parasite receptors, while sparing mammalian receptors, underlies its safety in animals.
DOSAGE AND ADMINISTRATION
Selamectin is administered topically once a month at a recommended dose of 6 mg/kg of body weight for both cats and dogs. The medication is applied directly to the skin, typically at the base of the neck or between the shoulder blades, where the animal cannot lick it off. Because oral absorption is poor, it is specifically formulated for topical use, and monthly application provides systemic protection against a broad spectrum of parasites, including fleas, heartworms, ear mites, sarcoptic mange, and certain ticks. For optimal absorption, the skin should be dry at the time of application, and animals should not be bathed or allowed to swim for at least 24–48 hours afterward. Care should be taken when using selamectin in very young, sick, or debilitated animals, and veterinary guidance is recommended for puppies or kittens under six weeks of age.
DRUG INTERACTIONS
Selamectin is generally considered safe and has few known major drug interactions, having been used alongside many medications during clinical studies without significant issues. However, certain drugs may interact with selamectin, potentially increasing the risk of side effects or altering its effectiveness. Possible interactions have been noted with some antifungal agents, such as itraconazole or ketoconazole, as well as with cyclosporine and erythromycin, though the precise effects are not always well-documented in pets. It is therefore essential to inform your veterinarian about all medications, supplements, vitamins, or herbal products your pet is taking to avoid potential complications.
FOOD INTERACTIONS
Selamectin has no known interactions with food, but it is important to inform your veterinarian about all medications, supplements, and herbal products your pet may be taking. While food does not affect selamectin, certain medications can interact with it, particularly in animals carrying genetic mutations such as the MDR1 gene. Therefore, veterinary guidance is essential to identify and manage any potential drug interactions.
CONTRAINDICATIONS
Selamectin is contraindicated in animals younger than six weeks, in cases of hypersensitivity to the drug, and, in some situations, in pregnant or lactating animals. It is also not recommended for sick, debilitated, or underweight animals, and caution is advised in certain dog breeds carrying the MDR1 gene mutation. Selamectin should be avoided in animals with untested heartworm infections, as it is not effective against adult heartworms.
SIDE EFFECTS
Skin and application site issues: Mild irritation, redness, hives, or hair loss at the application site.
Gastrointestinal: Vomiting or diarrhea.
Behavioral: Lethargy (tiredness) or decreased appetite.
Neurological: Muscle tremors, incoordination, or seizures.
Respiratory: Difficulty breathing or increased breathing rate.
Other: Generalized itching, fever, or a severe allergic reaction.
OVERDOSE
Muscle tremors or seizures
Lack of coordination or ataxia
Excessive drooling (hypersalivation)
Lethargy, weakness, or disorientation
Dilated pupils (in dogs)
Coma, in severe cases
Vomiting
Diarrhea
Loss of appetite
TOXICITY
Selamectin is generally well-tolerated in cats and dogs when used at the recommended dose. Toxicity is rare but can occur with accidental overdose or in animals with heightened sensitivity, such as those with the MDR1 gene mutation. Signs of toxicity may include vomiting, diarrhea, drooling, lethargy, tremors, uncoordinated movements, or, in severe cases, seizures. Large overdoses can also cause central nervous system depression. The primary approach to managing selamectin toxicity is supportive care, since no specific antidote exists. Toxic effects are more likely in very young, debilitated, or severely ill animals, highlighting the importance of proper dosing and veterinary supervision.
