Milbemycin Oxime — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.5-1.0 mg/kg (heartworm prevention); 2 mg/kg (treatment of demodectic mange, daily for 60-90 days)	Monthly for heartworm prevention; daily for mange treatment	Duration: Monthly for prevention; 60-90 days for mange Notes: Administer with food to enhance absorption. For heartworm prevention, give once monthly year-round or at least during mosquito season. For mange, higher doses may be used under veterinary supervision.
Cat	PO	0.5-1.0 mg/kg (heartworm prevention); 2 mg/kg (treatment of gastrointestinal nematodes)	Monthly for heartworm prevention; single dose or repeated as needed	Duration: Monthly for prevention; as directed for treatment Notes: Administer with food. Safety in kittens under 6 weeks of age has not been established.

Clinical Indications & Species Uses

General Indications

Prevention of heartworm disease in dogs and cats
Treatment of gastrointestinal nematodes in dogs and cats

Dog (Canine)

Prevention of heartworm disease (Dirofilaria immitis)
Treatment and control of adult hookworm (Ancylostoma caninum, Uncinaria stenocephala)
Treatment and control of adult roundworm (Toxocara canis, Toxascaris leonina)
Treatment and control of adult whipworm (Trichuris vulpis)
Treatment of demodectic mange (Demodex spp.)
Treatment of sarcoptic mange (Sarcoptes scabiei)

Cat (Feline)

Prevention of heartworm disease (Dirofilaria immitis)
Treatment and control of adult roundworm (Toxocara cati)
Treatment and control of adult hookworm (Ancylostoma tubaeforme)

Pharmacology & Mechanism of Action

Drug Class: Macrocyclic lactone | Pharmacological Group: Anthelmintic / Endectocide

Mechanism of Action: Milbemycin oxime is a macrocyclic lactone that binds to glutamate-gated chloride channels in nematode and arthropod nerve and muscle cells. This binding increases the permeability of the cell membrane to chloride ions, leading to hyperpolarization and paralysis of the parasite. It also interacts with GABA-gated chloride channels, enhancing the effect. The selective toxicity is due to the higher affinity of the drug for invertebrate channels compared to mammalian channels.

Pharmacodynamics: Milbemycin oxime is active against a broad spectrum of nematodes and some arthropods. It is effective against adult and larval stages of heartworm (Dirofilaria immitis), gastrointestinal nematodes such as hookworms (Ancylostoma spp.), roundworms (Toxocara spp., Toxascaris leonina), and whipworms (Trichuris vulpis). It also has activity against certain mites (e.g., Demodex spp., Sarcoptes scabiei) and is used for the treatment of demodectic mange in dogs. The drug is not effective against cestodes (tapeworms) or trematodes.

⚡ Pharmacokinetics Summary

Absorption: Milbemycin oxime is well absorbed after oral administration in dogs and cats. Peak plasma concentrations are reached within 2-4 hours. Absorption is enhanced when given with food.

Distribution: The drug is widely distributed throughout the body, with high concentrations in the liver, kidney, and adipose tissue. It crosses the blood-brain barrier to a limited extent, which contributes to its safety in mammals.

Metabolism: Milbemycin oxime is extensively metabolized in the liver, primarily by hydroxylation and oxidation. The metabolites are less active than the parent compound.

Excretion: The drug and its metabolites are excreted primarily in the feces via bile, with a smaller amount excreted in urine. In dogs, the elimination half-life is approximately 7-14 days, while in cats it is around 24-48 hours.

Half-Life: Dog: 7-14 days; Cat: 24-48 hours

Bioavailability: Oral bioavailability is approximately 80-90% in dogs and cats, with food increasing absorption.

Protein Binding: Approximately 90% bound to plasma proteins in dogs.

Available Formulations & Strengths

Oral Tablet 2.3 mg, 5.75 mg, 11.5 mg, 23 mg (for dogs); 1.25 mg, 5.75 mg (for cats) (PO)

Oral Tablet (combination with praziquantel) Milbemycin oxime 2.5 mg / praziquantel 25 mg; 5 mg / 50 mg; 10 mg / 100 mg; 20 mg / 200 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to milbemycin oxime or any component of the formulation
Use in dogs or cats with known hypersensitivity to macrocyclic lactones
Do not use in puppies younger than 4 weeks of age or kittens younger than 6 weeks of age
Do not use in breeding animals unless specifically indicated and under veterinary supervision

Warnings & Clinical Precautions:

Use with caution in dogs with a history of seizures or neurological disorders, as macrocyclic lactones may lower the seizure threshold.
In heartworm-positive dogs, treatment with milbemycin oxime may cause a hypersensitivity reaction due to the rapid killing of microfilariae. Monitor closely and consider pretreatment with antihistamines or corticosteroids.
Do not use in severely debilitated animals or those with significant hepatic or renal impairment without careful risk-benefit assessment.
Keep out of reach of children and pets.
Use with caution in Collies and other breeds with the MDR1 (ABCB1) gene mutation, as they may be more sensitive to the neurological effects of macrocyclic lactones.

Adverse Effects & Reactions

Common:

Vomiting
Diarrhea
Lethargy
Decreased appetite

Serious / Severe:

Neurological signs (ataxia, tremors, seizures) especially in sensitive breeds or at high doses
Hypersensitivity reactions (facial swelling, urticaria, pruritus) in heartworm-positive dogs
Bone marrow suppression (rare)

Rare:

Allergic reactions (anaphylaxis)
Hepatotoxicity
Pancytopenia

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Ivermectin and other macrocyclic lactones	Additive toxicity may occur, increasing the risk of neurological adverse effects.	High
P-glycoprotein inhibitors (e.g., ketoconazole, itraconazole, cyclosporine)	May increase plasma concentrations of milbemycin oxime, potentially leading to toxicity.	Moderate
CYP450 inhibitors (e.g., cimetidine)	May decrease metabolism of milbemycin oxime, increasing drug levels.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Diarrhea
Ataxia
Tremors
Mydriasis
Depression
Seizures
Coma

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Induce emesis if ingestion is recent and the animal is conscious. Administer activated charcoal to reduce absorption. Provide intravenous fluids, anticonvulsants (e.g., diazepam) for seizures, and supportive care. Monitor vital signs and neurological status. In severe cases, mechanical ventilation may be necessary.

Food Animal Withdrawal Times

Not approved for use in food-producing animals. Do not use in animals intended for human consumption.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20°C to 25°C (68°F to 77°F), with excursions permitted between 15°C and 30°C (59°F and 86°F).

Handling & Special Conditions: Protect from moisture. Keep in original container with the lid tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved by the FDA for use in dogs and cats (e.g., Interceptor, Sentinel).

Extra-Label (Off-Label) Use: In the United States, milbemycin oxime is approved for use in dogs and cats. Extra-label use in other species is permitted under the Animal Medicinal Drug Use Clarification Act (AMDUCA) only by or on the order of a licensed veterinarian within a valid veterinarian-client-patient relationship, and only when no approved animal drug is available. Use in food animals is prohibited due to lack of withdrawal times.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Milbemycin oxime is a broad-spectrum anthelmintic commonly used for heartworm prevention and control of intestinal parasites in dogs and cats. It is generally well-tolerated, but caution is advised in breeds with MDR1 mutations. For heartworm prevention, it should be administered monthly, ideally year-round. In heartworm-positive animals, treatment may cause a hypersensitivity reaction; therefore, testing for heartworm before starting prevention is recommended. For treatment of demodectic mange, higher doses and prolonged therapy are required, and concurrent management of secondary bacterial infections is important. Always administer with food to maximize absorption. Monitor for adverse effects, especially neurological signs, in sensitive animals.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)