Fenbendazole

Mechanism of action

Fenbendazole works by binding to tubulin, a protein that is part of the microtubules in the cells of parasites. This binding disrupts the microtubules' formation and function, leading to the parasites' inability to absorb nutrients, resulting in their eventual death. This mode of action makes fenbendazole effective against both adult and larval stages of many parasitic worms.

Uses in veterinary medicine

Dogs and cats

Fenbendazole is commonly used to treat intestinal parasites, including roundworms, hookworms, whipworms, and certain tapeworms. It is often administered as part of a broader deworming protocol.

Horses

In equine medicine, fenbendazole is used to control strongyles, pinworms, and ascarids. It is available in paste form for easy administration.

Cattle and goats

Fenbendazole is effective against lungworms, stomach worms, and intestinal worms in ruminants. It is administered through feed, drenching, or bolus form.

Drug interactions

Drug interactions may occur if salicylanilides such as dibromsalan and niclosamide are co-administered. Abortions in cattle and death in sheep have been reported after using these medications together. Abortions in domestic ruminants have been associated with concurrent use of antitrematode therapeutic agents.

Toxicity

Fenbendazole is poorly absorbed from the gastrointestinal tract in most species. The in laboratory animals exceeds 10 g/kg when administered orally.

Metabolism

Fenbendazole is metabolized in the liver to oxfendazole, which is anthelmintic, too; oxfendazole partially gets reduced back to fenbendazole in the liver and rumen. Also, fenbendazole itself is an active metabolite of another anthelmintic drug, febantel.

Safety and precautions

Fenbendazole is generally safe when used as directed, but following veterinary guidelines and dosage instructions to avoid potential side effects is important. Some animals may experience mild gastrointestinal upset. It is not recommended for use in pregnant animals without veterinary advice. In treatment in dogs that go on for longer than the label recommendation, there have been reports of bone marrow hypoplasia and pancytopenia occurring.

Two case reports describe dangerous liver injury in human patients taking fenbendazole off-label. When they stopped taking the drug, the liver injury resolved.

Research directions

Fenbendazole has been proposed for repurposing as an anticancer drug due to its ability to disrupt energy metabolism in cancer cells and induce cell death through multiple mechanisms. Early results with fenbendazole and the related drug mebendazole are somewhat promising, from in vitro studies, animal studies and a few case reports from people who took it off-label. Fenbendazole's poor bioavailability when administered orally, though, remains a challenge for its use as a cancer treatment. Some patients taking fenbendazole off-label without a doctor's supervision suffered potentially dangerous liver dysfunction, although this resolved after stopping taking the drug.

Exaggerated reports have circulated on social media that fenbendazole is a "miracle cure" for cancers, often with the use of florid testimonials. This information, however, is often less conclusive than it appears, such as reports from patients who were taking fenbendazole alongside conventional drugs, radiation and/or surgery, making it impossible to tell what caused their recovery.

References

References

1. (1 September 2024). "Oral Fenbendazole for Cancer Therapy in Humans and Animals". Anticancer Research.
2. (2025). "NanoDSF Screening for Anti-tubulin Agents Uncovers New Structure–Activity Insights". Journal of Medicinal Chemistry.
3. (February 2013). "Fenbendazole as a potential anticancer drug". Anticancer Research.
4. (August 2018). "Fenbendazole acts as a moderate microtubule destabilizing agent and causes cancer cell death by modulating multiple cellular pathways". Scientific Reports.
5. "Fenbendazole (Panacur®, Safe-guard®) for Dogs and Cats".
6. "What Is Fenbendazole and Is FenCare Right for My Horse? {{!}} Farnam".
7. "Fenbendazole".
8. (2005). "Plumb's veterinary drug handbook". PhrmaVet.
9. (March 2021). "Infectious bovine abortions: observations from an organized dairy herd". Brazilian Journal of Microbiology.
10. (2015-07-26). "Fenbendazole, Anthelmintic for Veterinary Use on Cattle, Sheep, Goats, Pig, Poultry, Horses, Dogs and Cats Against Roundworms and Tapeworms".
11. (2015-07-26). "Oxfendazole, Anthelmintic for Veterinary Use on Cattle, Sheep, Goats, Horses, Dogs and Cats Against Roundworms and Tapeworms".
12. (2015-07-26). "Febantel for Veterinary Use on Dogs, Cats, Cattle, Sheep, Goats, Pig and Poultry Against Roundworms and Tapeworms".
13. "Prescription Label".
14. Medicine, Center for Veterinary. (2024-04-08). "Dear Veterinarian Letter regarding adverse events associated with extra-label use of fenbendazole in dogs". FDA.
15. (2021). "Drug-Induced Liver Injury in a Patient with Nonsmall Cell Lung Cancer after the Self-Administration of Fenbendazole Based on Social Media Information". Case Reports in Oncology.
16. (October 2023). "S3634 Colon Cleanse Revenge Fenbendazole Causes Severe Drug-Induced Liver Injury". American Journal of Gastroenterology.
17. (September 2024). "Oral Fenbendazole for Cancer Therapy in Humans and Animals". Anticancer Research.
18. (August 2019). "Mebendazole as a Candidate for Drug Repurposing in Oncology: An Extensive Review of Current Literature". Cancers.
19. (4 March 2024). "Fenbendazole is fast becoming the laetrile of the 2020s".