How does fenbendazole work against parasites?

Fenbendazole binds selectively to parasite beta-tubulin with approximately 250–400 times higher affinity than mammalian tubulin. This binding inhibits the polymerization of tubulin into microtubules, which are essential for multiple cellular functions: nutrient absorption (blocking glucose uptake starves the parasite), intracellular transport, mitosis, and maintenance of cell shape. The selectivity for parasite over host tubulin is what gives fenbendazole its excellent safety margin — animals can tolerate doses many times higher than the therapeutic dose without adverse effects.

What parasites does fenbendazole cover that other dewormers miss?

Fenbendazole has a notably broad spectrum that includes three groups many anthelmintics do not cover simultaneously: (1) lungworms (Dictyocaulus viviparus in cattle, D. filaria in sheep) — ivermectin covers these but praziquantel does not; (2) tapeworms (Moniezia species in ruminants, Taenia pisiformis in dogs) — benzimidazoles are one of few classes with consistent cestode activity; (3) Giardia — fenbendazole at 50 mg/kg for 3–5 days is a standard treatment for giardiasis in dogs, a use not shared by ivermectin or pyrantel. This triple coverage (nematodes + lungworms + cestodes) makes fenbendazole valuable in mixed-species grazing operations.

Fenbendazole | CAS 43210-67-9 | Veterinary API Supplier China

Name: Fenbendazole
SKU: 43210-67-9
Availability: InStock

Quick Facts

CAS Number	43210-67-9
Molecular Formula	C₁₅H₁₃N₃O₂S
Molecular Weight	299.35
Pharmacopoeia	EP
Packaging	25 KG/DRUM

GMP Certified EP Standard

Product Overview

Fenbendazole was developed by Hoechst AG (now part of Sanofi) in the 1970s as part of a systematic benzimidazole optimization program that began with the discovery of thiabendazole at Merck in 1961. The benzimidazole carbamate scaffold — a bicyclic ring system with a carbamate side chain — represented a breakthrough in anthelmintic pharmacology because it offered selective toxicity against parasitic worms without affecting the host animal. Hoechst's chemists modified the substituents on the benzimidazole core to optimize spectrum, potency, and safety, eventually producing fenbendazole [5-(phenylthio)-1H-benzimidazol-2-yl]carbamic acid methyl ester. The phenylthio substitution at position 5 proved crucial for broadening activity against lungworms and cestodes beyond what earlier benzimidazoles could achieve.

Fenbendazole's mechanism is elegantly selective: it binds to parasite beta-tubulin with approximately 250–400 times higher affinity than mammalian tubulin. This binding prevents tubulin dimer polymerization into microtubules — the structural proteins essential for nutrient absorption (especially glucose uptake), intracellular transport, cell division, and maintenance of cellular architecture. The parasite essentially starves to death while the host's microtubule-dependent processes continue unaffected. This selectivity ratio explains fenbendazole's remarkable safety margin — doses 100 times the therapeutic level have been administered to cattle without clinical toxicity.

One of fenbendazole's most valuable clinical attributes is its cross-species breadth — it is one of very few anthelmintics with labeled indications across five major target species groups: cattle, sheep/goats, swine, poultry, and companion animals (dogs and cats). This versatility makes it a cornerstone API for veterinary pharmaceutical companies seeking to cover multiple formulation types from a single active ingredient. Fenbendazole is also valued for its activity against hypobiotic (arrested) larvae of Ostertagia ostertagi in cattle — a key epidemiological target for strategic deworming programs — and for its efficacy against Giardia in dogs, a protozoal application not shared by other benzimidazoles.

As of mid-2026, the global fenbendazole market is driven by sustained demand across all major livestock and companion animal segments. The companion animal deworming segment shows particularly strong growth, driven by increasing pet ownership and veterinary expenditure in North America, Europe, and urbanizing Asia-Pacific markets. China and India are the dominant API producers. KingWish supplies GMP-certified fenbendazole meeting EP standards, supporting veterinary pharmaceutical manufacturers producing oral suspensions, tablets, feed premixes, and paste formulations worldwide.

Quality Specifications

Assay (HPLC)	98.0%–102.0% on dried basis, per EP monograph
Related Substances	Impurity A (oxfendazole) ≤ 0.5%; Impurity B ≤ 0.3%; any other ≤ 0.10%; total ≤ 1.0%
Loss on Drying	≤ 1.0% (100–105°C)
Sulfated Ash	≤ 0.1%
Heavy Metals	≤ 20 ppm
GMP Status	Manufactured under ICH Q7 GMP conditions

Veterinary Applications

Ruminant Deworming

Oral drench at 5–7.5 mg/kg for cattle, 5 mg/kg for sheep/goats. Feed premix for group treatment. Effective against GI roundworms, lungworms, tapeworms, and hypobiotic Ostertagia larvae.

Companion Animal Formulations

Tablets/granules at 50 mg/kg for 3 days (dogs) or 5 days (cats). Oral paste for horses at 5–10 mg/kg. Effective against roundworms, hookworms, whipworms, and Giardia.

Therapeutic Areas

Cattle: GI nematodes (Ostertagia, Haemonchus, Trichostrongylus, Cooperia), lungworms, tapeworms, inhibited larvae
Sheep & Goats: Broad-spectrum control of GI and respiratory parasites
Swine: Ascaris suum, Oesophagostomum, Trichuris suis
Poultry: Ascaridia galli, Heterakis gallinarum, Capillaria species
Companion Animals: Roundworms, hookworms, whipworms, Taenia tapeworms, Giardia

Sourcing Fenbendazole API: Key Checks

Documentation

Request CoA with full HPLC chromatogram showing impurity A (oxfendazole — the sulfoxide metabolite) and impurity B levels. Confirm loss on drying and sulfated ash are within EP limits. For oral suspension formulations, request particle size distribution (micronized grades preferred).

Red Flags

Elevated impurity A (oxfendazole >0.5%) indicates oxidative degradation during synthesis or storage. Color change from white to off-white/yellow suggests degradation. Wide particle size distribution can cause content uniformity issues in suspensions and pastes.

Packaging & Logistics

Standard: 25 KG/DRUM with double LDPE liner. Store at room temperature, protected from light and moisture. Product is stable under normal conditions. Micronized and standard grades available.

Market Context

Global fenbendazole market driven by livestock deworming demand (particularly cattle and sheep) and growing companion animal segment. Branded products (Panacur, Safe-Guard) dominate US/European markets. China and India are leading API producers. Market report

Frequently Asked Questions

Fenbendazole binds selectively to parasite beta-tubulin with approximately 250–400 times higher affinity than mammalian tubulin. This binding inhibits the polymerization of tubulin into microtubules, which are essential for multiple cellular functions: nutrient absorption (blocking glucose uptake effectively starves the parasite), intracellular transport, mitosis, and maintenance of cell shape. The selectivity for parasite over host tubulin is what gives fenbendazole its excellent safety margin — animals can tolerate doses many times higher than therapeutic levels without adverse effects.

Fenbendazole has a notably broad spectrum that includes three groups many anthelmintics do not cover simultaneously: (1) lungworms (Dictyocaulus viviparus in cattle, D. filaria in sheep) — ivermectin covers these but praziquantel does not; (2) tapeworms (Moniezia species in ruminants, Taenia pisiformis in dogs) — benzimidazoles are one of the few classes with consistent cestode activity alongside praziquantel; (3) Giardia — fenbendazole at 50 mg/kg for 3–5 days is a standard treatment for giardiasis in dogs, a use not shared by ivermectin, pyrantel, or praziquantel.

All three are benzimidazole carbamates with the same mechanism (beta-tubulin binding), but they differ in spectrum, metabolism, and regulatory status: Fenbendazole is approved exclusively for veterinary use with the broadest species label — cattle, sheep, goats, swine, horses, poultry, dogs, cats, and zoo animals. Albendazole is approved for both human (hydatid disease, neurocysticercosis) and veterinary use, with somewhat higher systemic absorption and greater teratogenicity risk in early pregnancy. Mebendazole is primarily a human anthelmintic (pinworm, roundworm, hookworm) with limited veterinary applications. Fenbendazole is metabolized in the liver to oxfendazole (its sulfoxide, also an active anthelmintic) and fenbendazole sulfone, with parent drug and metabolites excreted primarily in feces.

Critical EP specifications for fenbendazole API: (1) assay by HPLC not less than 98.0% and not more than 102.0% on dried basis; (2) related substances — impurity A (fenbendazole sulfoxide/oxfendazole) not more than 0.5%, impurity B not more than 0.3%, any other individual impurity not more than 0.10%, total impurities not more than 1.0%; (3) loss on drying not more than 1.0% at 100–105°C; (4) sulfated ash not more than 0.1%; (5) heavy metals not more than 20 ppm. For oral suspension and paste formulations, particle size distribution is critical — micronized grades with D90 below 10 um provide significantly better oral bioavailability than standard-grade material.

Standards: EP | CAS: 43210-67-9 | Quality data verified against pharmacopoeia monograph COA and MSDS available | DrugBank | July 2026