Fenbendazole (Panacur)

Benzimidazole anthelmintic Anthelmintic

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 50 mg/kg q24h Duration: 3 consecutive days
Notes: For giardiasis, use 50 mg/kg q24h for 3-5 days. For lungworms, 50 mg/kg q24h for 5-7 days.
Cat PO 50 mg/kg q24h Duration: 3 consecutive days
Notes: For lungworms, 50 mg/kg q24h for 5 days.
Horse PO 5-10 mg/kg single dose or q24h Duration: 1-5 days depending on parasite
Notes: For encysted small strongyles, use 10 mg/kg q24h for 5 days.
Cattle PO 5-10 mg/kg single dose Duration: 1 day
Notes: For lungworms, 5 mg/kg single dose. For tapeworms, 10 mg/kg.
Sheep/Goat PO 5-10 mg/kg single dose Duration: 1 day
Notes: For tapeworms, 10 mg/kg.
Rabbit PO 20-50 mg/kg q24h Duration: 3-5 days
Notes: For E. cuniculi, 20 mg/kg q24h for 28 days.
Poultry PO 10-20 mg/kg q24h Duration: 3-5 days
Notes: Administer in feed or water.

Clinical Indications & Species Uses

General Indications
  • Treatment and control of susceptible internal parasites
Dog (Canine)
  • Gastrointestinal nematodes (roundworms, hookworms, whipworms)
  • Tapeworms (Taenia spp.)
  • Lungworms (Oslerus osleri, Filaroides spp.)
  • Giardiasis (off-label)
Cat (Feline)
  • Gastrointestinal nematodes (roundworms, hookworms)
  • Lungworms (Aelurostrongylus abstrusus)
  • Tapeworms (Taenia spp.)
Horse (Equine)
  • Large strongyles (Strongylus spp.)
  • Small strongyles (Cyathostomins)
  • Ascarids (Parascaris equorum)
  • Pinworms (Oxyuris equi)
  • Lungworms (Dictyocaulus arnfieldi)
Cattle (Bovine)
  • Gastrointestinal nematodes (Ostertagia, Cooperia, Haemonchus, Trichostrongylus)
  • Lungworms (Dictyocaulus viviparus)
  • Tapeworms (Moniezia spp.)
Small Ruminants (Sheep / Goat)
  • Gastrointestinal nematodes (Haemonchus, Teladorsagia, Trichostrongylus)
  • Lungworms (Dictyocaulus filaria)
  • Tapeworms (Moniezia spp.)
Rabbit & Small Mammals
  • Gastrointestinal nematodes (Passalurus ambiguus)
  • Encephalitozoon cuniculi (off-label)
Avian & Poultry
  • Ascarids (Ascaridia spp.)
  • Capillaria spp.
  • Tapeworms (Raillietina spp.)
Exotic & Other Species
  • Reptiles: gastrointestinal nematodes
  • Small mammals (ferrets, guinea pigs): nematodes

Pharmacology & Mechanism of Action

Drug Class: Benzimidazole anthelmintic | Pharmacological Group: Anthelmintic

Mechanism of Action: Fenbendazole binds to beta-tubulin, inhibiting microtubule polymerization in parasitic cells. This disrupts glucose uptake and energy metabolism, leading to depletion of glycogen stores and eventual death of the parasite. It is effective against various nematodes, cestodes, and some protozoa.

Pharmacodynamics: Fenbendazole exhibits a broad spectrum of anthelmintic activity. It is ovicidal and larvicidal against many gastrointestinal nematodes, and also effective against lungworms, tapeworms, and certain tissue-dwelling larvae. Its action is slow, requiring multiple doses for optimal efficacy.

⚑ Pharmacokinetics Summary

Absorption: Fenbendazole is poorly absorbed from the gastrointestinal tract; systemic absorption is low (less than 10% in most species). Absorption is enhanced when given with food, especially fatty meals.
Distribution: Due to low systemic absorption, distribution is mainly within the gastrointestinal tract. The small amount absorbed is distributed to tissues, with higher concentrations in liver and fat.
Metabolism: Fenbendazole is extensively metabolized in the liver, primarily to oxfendazole (fenbendazole sulfoxide) and fenbendazole sulfone. These metabolites also have anthelmintic activity.
Excretion: The majority of the drug is excreted in feces as unchanged drug or metabolites. A small fraction is excreted in urine. In ruminants, some metabolites are excreted in milk.
Half-Life: The elimination half-life in dogs is approximately 10-15 hours; in cattle, it is about 10-20 hours; in horses, about 10-15 hours.
Bioavailability: Oral bioavailability is low, typically less than 10% in most species, but can increase to 30-50% when administered with food.
Protein Binding: Protein binding is moderate, approximately 60-70% in plasma.

Available Formulations & Strengths

Oral Suspension 10% (100 mg/mL) (PO)
Oral Paste 10% (100 mg/g) (PO)
Granules/Powder 22.2% (222 mg/g) (PO)
Tablets 222 mg, 400 mg, 1 g (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to fenbendazole or other benzimidazoles
  • Do not use in animals with known liver disease (caution)
  • Not for use in lactating dairy cattle (in some countries, check label)
Warnings & Clinical Precautions:
  • Use with caution in debilitated animals or those with severe parasitic burden.
  • In horses, do not use in pregnant mares unless benefit outweighs risk (safe in most cases).
  • In cattle, observe withdrawal times for meat and milk.
  • May cause mild gastrointestinal upset in some animals.
  • Do not use in animals with known hypersensitivity.
  • For extra-label use, follow AMDUCA regulations.

Adverse Effects & Reactions

Common:

  • Mild vomiting
  • Diarrhea
  • Anorexia (rare)

Serious / Severe:

  • Bone marrow suppression (rare, with high doses)
  • Hepatotoxicity (rare)

Rare:

  • Allergic reactions
  • Teratogenic effects (reported in some species at high doses)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Cimetidine May increase plasma levels of fenbendazole by inhibiting its metabolism. Moderate
Ivermectin Potential for additive effects; generally safe but monitor for toxicity. Mild
Praziquantel No significant interaction; often used in combination. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Vomiting
  • Diarrhea
  • Ataxia
  • Depression
  • In severe cases, seizures or liver failure

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Induce vomiting if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide fluid therapy and supportive care. Monitor liver enzymes and blood counts.

Food Animal Withdrawal Times

πŸ₯© Meat: 8 daysπŸ₯› Milk: 4 days

Withdrawal times vary by country and formulation. For cattle, meat withdrawal is 8 days and milk withdrawal is 4 days. For poultry, eggs withdrawal is not established; consult label. For sheep/goats, meat withdrawal is 6 days (check local regulations).

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 15-30Β°C (59-86Β°F)

Handling & Special Conditions: Protect from freezing. Keep container tightly closed. Store in a dry place.

Dispensing Status: Over-The-Counter (OTC)

Approval Status: Approved for use in dogs, cats, horses, cattle, and sheep. Not approved for poultry in the US, but extra-label use is possible.

Extra-Label (Off-Label) Use: Extra-label use in food animals requires veterinary oversight and extended withdrawal times. Follow AMDUCA regulations.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Fenbendazole is a broad-spectrum anthelmintic widely used in veterinary practice. It is generally safe and well-tolerated. For optimal efficacy, ensure accurate dosing and complete the full course. In cases of high parasite burden, consider concurrent use of other anthelmintics with different mechanisms to reduce resistance. In food animals, adhere to withdrawal times. In horses, fenbendazole is often used for encysted small strongyles with a 5-day regimen. In dogs and cats, it is commonly used for giardiasis off-label. Always confirm diagnosis and parasite susceptibility before treatment.

References & Literature

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