Fenbendazole
Methyl [5-(phenylthio)-1H-benzimidazol-2-yl]carbamate
Also known as: Panacur · Safe-Guard · Granofen · Wormazole · Hoe-881V · Fenbendazolum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceEncephalitozoon cuniculi infection after diagnostic risk-benefit discussion
- q24h · 30d
Governing clinical context (9)
- Infections marked by ocular, CNS, and renal disease
- Otitis interna and/or media
- Most common cause of vestibular signs must exclude first
- CNS infections (toxoplasmosis, abscess), neoplasia, trauma
- Many North American and European rabbits will be positive, indicative of prior (in utero) exposure and not necessarily active disease
- Definitive diagnosis based on organism (obligate intracellular microsporidian parasite) identification in affected tissues, generally postmortem
- Head CT/MRI is considered for excluding bulla disease, abscess, neoplasia
- Positive serology and response to treatment supports presumed diagnosis, but risk/benefit discussion required prior to initiation
- Risk of bone marrow suppression/aplastic anemia/pancytopenia with therapy must be considered/discussed prior to therapy
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Fenbendazole is a highly effective, broad-spectrum benzimidazole anthelmintic widely used in veterinary medicine across multiple species.
Key Clinical Features:
- Nematode Efficacy: Highly effective against gastrointestinal roundworms, hookworms, and whipworms.
- Cestode Efficacy: Effective against Taenia species tapeworms, but not effective against the common flea tapeworm (Dipylidium caninum).
- Protozoal Efficacy: Considered a first-line treatment for Giardia infections in dogs and cats.
- Respiratory Parasites: Frequently utilized for various lungworm infections (e.g., Aelurostrongylus, Capillaria, Paragonimus).
Clinical Pearl: Fenbendazole has an exceptionally wide margin of safety and is considered the drug of choice for treating parasitic infections, including Giardia, in pregnant animals.
Mechanism of action
Fenbendazole is a time-dependent antiparasitic agent, which is why multi-day dosing is typically required in small animals.
- Primary Mechanism: It selectively binds to parasite β-tubulin → prevents tubulin polymerization → inhibits microtubule formation. This disrupts essential intracellular transport mechanisms within the parasite.
- Secondary Mechanism: At higher concentrations, it disrupts metabolic pathways by inhibiting key enzymes such as fumarate reductase and malate dehydrogenase → depletes ATP/energy reserves → parasite death.
Safety & warnings
Contraindications
- Not FDA-approved for use in horses intended for food purposes
Adverse effects
- Hypersensitivity reactions (secondary to antigen release by dying parasites)
- Salivation (infrequent)
- Vomiting (infrequent)
- Diarrhea (infrequent)
- Pancytopenia (rarely reported in dogs)
- Bone marrow hypoplasia (rarely reported in dogs)
Precautions
Important: Single doses are generally not effective in dogs and cats; treatment must continue for at least 3 consecutive days.
- Hypersensitivity: Reactions may occur secondary to antigen release by dying parasites, particularly in animals with high parasite burdens.
- Food Animal Withdrawal: Observe appropriate slaughter withdrawal times for cattle and swine based on the specific product used.
Drug interactions
Abortions in cattle and death in sheep have been reported after concurrent use
Monitoring
- Fecal examinations to confirm parasite clearance
- Monitor for hypersensitivity reactions if treating heavy parasite burdens
- Fecal examination for parasite clearance
- Complete blood count (CBC) if bone marrow hypoplasia is suspected
Pharmacokinetics
Absorption
Marginally absorbed after oral administration. The amount absorbed from the gut is associated with the solubility of the drug, not the dose given. Bioavailability is significantly increased in dogs when administered with food (fat content does not alter this).
Distribution
Reaches systemic circulation sufficiently to treat migrating larvae and lungworms, though specific distribution parameters are not detailed.
Metabolism
Absorbed fenbendazole is metabolized to the active compound oxfendazole (sulfoxide) and the sulfone.
Elimination
In sheep, cattle, and pigs, 44-50% of a dose is excreted unchanged in the feces, and <1% in the urine.
Overdose
Fenbendazole is exceptionally well tolerated, even at doses up to 100X the recommended dose.
- The LD50 in laboratory animals exceeds 10 grams/kg when administered orally.
- It is highly unlikely that an acute overdosage would lead to any significant clinical signs.
Available products
Formulations
- Granules (22.2%)
- Paste (10%)
- Type B Medicated Feed (1.8%, 0.96%)
- Type C Medicated Feed (0.50%, 35% Salt Free-choice)
- Pellets (0.5%, 1.96%)
- Premix 20% Type A
- Chewable Tablets (Combination with Ivermectin and Praziquantel)
- 222 mg/g oral granules (22%)
- 440 mg/g oral granules (44%)
- 880 mg/g oral granules (88%)
- 25 mg/ml oral suspension (2.5%)
- 100 mg/ml oral suspension (10%)
- 187.5 mg/g oral paste (18.75%)
Veterinary
- Fenbendazole Granules: 222 mg/gram (22.2%)
- Fenbendazole Suspension: 100 mg/mL (10%)
- Fenbendazole Paste: 100 mg/gram (10%)
- Fenbendazole Type B Medicated Feed (1.8%, 0.96%)
- Fenbendazole Type C Medicated Feed (0.50%, 35% Salt Free-choice)
- Fenbendazole Pellets (0.5%, 1.96%)
- Fenbendazole Premix 20% Type A (200 mg/gram)
- Combination Products: Fenbendazole/Ivermectin/Praziquantel Chewable Tablets (Panacur Plus Soft Chews)
- Granofen
- Wormazole
Regulatory status
Authorized proprietary products available.
Available as AVM-GSL (General Sales List) and NFA-VPS (Non-Food Animal - Veterinarian, Pharmacist, SQP).
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Fenbendazole products should be stored at room temperature.
Client information
Important: In dogs, always give this medication with food to significantly improve its absorption and effectiveness.
- Multi-Day Dosing: Unlike some single-dose dewormers, fenbendazole usually requires 3 to 5 consecutive days of treatment to be effective. Do not skip doses.
- Safety: It is very safe, but occasionally pets may experience mild vomiting, drooling, or diarrhea.
- What to Expect: You may see dead worms in your pet's stool after treatment; this is normal.
- Giardia Treatment: If treating for Giardia, ensure you also bathe your pet (especially the hindquarters) and thoroughly clean their environment to prevent reinfection.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.