Mebendazole

Anthelmintic Benzimidazole

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 22 mg/kg q24h Duration: 3 days for roundworms/hookworms; 5 days for whipworms
Notes: Administer with food to enhance absorption.
Cat PO 22 mg/kg q24h Duration: 3 days
Notes: Use with caution in kittens under 6 weeks.
Horse PO 8.8 mg/kg q24h Duration: 5 days
Notes: For strongyles and ascarids.
Cattle PO 10-15 mg/kg q24h Duration: 3-5 days
Notes: Not commonly used; other benzimidazoles preferred.
Sheep/Goat PO 10-15 mg/kg q24h Duration: 3-5 days
Notes: Not commonly used; fenbendazole preferred.
Rabbit PO 50 mg/kg q24h Duration: 5 days
Notes: For pinworms.
Poultry PO 20-30 mg/kg q24h Duration: 3-5 days
Notes: Not approved in many countries; use with caution.

Clinical Indications & Species Uses

General Indications
  • Broad-spectrum anthelmintic for gastrointestinal nematodes and some cestodes
Dog (Canine)
  • Treatment of roundworms (Toxocara canis, Toxascaris leonina)
  • Hookworms (Ancylostoma caninum, Uncinaria stenocephala)
  • Whipworms (Trichuris vulpis)
  • Tapeworms (Taenia spp.)
Cat (Feline)
  • Treatment of roundworms (Toxocara cati)
  • Hookworms (Ancylostoma tubaeforme)
  • Tapeworms (Taenia taeniaeformis)
Horse (Equine)
  • Treatment of large strongyles (Strongylus spp.)
  • Small strongyles (Cyathostomes)
  • Roundworms (Parascaris equorum)
  • Pinworms (Oxyuris equi)
Cattle (Bovine)
  • Treatment of gastrointestinal nematodes (Ostertagia, Cooperia, Haemonchus)
  • Lungworms (Dictyocaulus viviparus)
Small Ruminants (Sheep / Goat)
  • Treatment of gastrointestinal nematodes (Teladorsagia, Haemonchus, Trichostrongylus)
Rabbit & Small Mammals
  • Treatment of pinworms (Passalurus ambiguus)
  • Roundworms (Baylisascaris procyonis)
Avian & Poultry
  • Treatment of roundworms (Ascaridia spp.)
  • Capillaria spp.
Exotic & Other Species
  • Treatment of nematodes in reptiles (e.g., oxyurids)
  • Some cestodes in exotic mammals

Pharmacology & Mechanism of Action

Drug Class: Anthelmintic | Pharmacological Group: Benzimidazole

Mechanism of Action: Mebendazole is a benzimidazole anthelmintic that exerts its effect by binding to tubulin, thereby inhibiting microtubule polymerization. This disrupts glucose uptake and depletes glycogen stores in susceptible parasites, leading to their immobilization and death. It is effective against a broad spectrum of nematodes and some cestodes.

Pharmacodynamics: Mebendazole is a broad-spectrum anthelmintic with activity against gastrointestinal nematodes (e.g., roundworms, hookworms, whipworms) and some tapeworms. It is ovicidal and larvicidal in some species. Its selective toxicity is due to higher affinity for parasite tubulin compared to mammalian tubulin.

⚑ Pharmacokinetics Summary

Absorption: Mebendazole is poorly absorbed from the gastrointestinal tract; systemic absorption is enhanced when taken with a fatty meal. In dogs, oral bioavailability is approximately 20-30%.
Distribution: The drug is widely distributed in tissues, with highest concentrations in the liver and bile. It crosses the blood-brain barrier poorly. It is highly bound to plasma proteins.
Metabolism: Mebendazole undergoes extensive first-pass metabolism in the liver, primarily by cytochrome P450 enzymes (CYP3A4), to inactive metabolites.
Excretion: The drug and its metabolites are excreted mainly in bile and feces. A small fraction is excreted in urine. In ruminants, a significant portion may be excreted in milk.
Half-Life: Dog: 2-6 hours; Cat: 2-4 hours; Horse: 2-3 hours; Cattle: 2-4 hours
Bioavailability: Oral: 20-30% in dogs; increased with food.
Protein Binding: Approximately 90-95% bound to plasma proteins.

Available Formulations & Strengths

Oral Tablet 100 mg, 500 mg (PO)
Oral Suspension 100 mg/5 mL (PO)
Powder for Oral Solution Varies (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to mebendazole or other benzimidazoles
  • Pregnancy (especially first trimester) unless benefits outweigh risks
  • Hepatic impairment
  • Concurrent use with metronidazole (risk of Stevens-Johnson syndrome)
Warnings & Clinical Precautions:
  • Use with caution in debilitated animals or those with severe malnutrition.
  • Do not use in animals with known liver disease.
  • In food animals, observe withdrawal times.
  • May cause bone marrow suppression with prolonged use.
  • Use in cats and dogs only when other anthelmintics are not available.

Adverse Effects & Reactions

Common:

  • Vomiting
  • Diarrhea
  • Anorexia
  • Lethargy

Serious / Severe:

  • Bone marrow suppression (neutropenia, thrombocytopenia)
  • Hepatotoxicity
  • Stevens-Johnson syndrome (rare)
  • Teratogenic effects

Rare:

  • Alopecia
  • Fever
  • Eosinophilia

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Cimetidine May increase mebendazole plasma levels by inhibiting its metabolism. Moderate
Metronidazole Increased risk of Stevens-Johnson syndrome and toxic epidermal necrolysis. High
Phenytoin May decrease mebendazole levels due to enzyme induction. Mild
Carbamazepine May decrease mebendazole efficacy. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Vomiting
  • Diarrhea
  • Abdominal pain
  • Hepatotoxicity
  • Bone marrow suppression

Emergency Treatment Protocol: Induce vomiting if recent ingestion. Administer activated charcoal. Provide supportive care (IV fluids, antiemetics). Monitor liver function and blood counts. There is no specific antidote.

Food Animal Withdrawal Times

πŸ₯© Meat: 14 daysπŸ₯› Milk: 4 days

Withdrawal times vary by country and formulation. Consult local regulations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (15-30Β°C)

Handling & Special Conditions: Keep container tightly closed. Protect from moisture.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs and cats in some countries; not FDA-approved for veterinary use in the US (human drug).

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited unless under a valid VCPR and with extended withdrawal times. Not approved for use in cattle, sheep, goats, or poultry in many countries.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Mebendazole is a broad-spectrum anthelmintic that is less commonly used in veterinary medicine due to the availability of safer and more effective benzimidazoles like fenbendazole. It is poorly absorbed systemically, making it effective for gastrointestinal parasites but less useful for systemic infections. It is contraindicated in pregnancy and should be used with caution in animals with hepatic disease. In food animals, it is not commonly used due to withdrawal times and lack of approval. For most veterinary applications, fenbendazole or other anthelmintics are preferred.

References & Literature

  • πŸ“š Plumb's Veterinary Drug Handbook
  • πŸ“š Papich Veterinary Pharmacology and Therapeutics
  • πŸ“š Compendium of Veterinary Products (CVP)