Dear Veterinarian,

For the past several years, the U.S. Food and Drug Administration’s Center for Veterinary Medicine has been monitoring reported cases of bone marrow hypoplasia and pancytopenia associated with extra-label use (i.e., treating longer than the labeled duration) of fenbendazole products in dogs (Panacur C, Safeguard Canine, and Panacur Granules 22.2%).  As of October 20, 2023, FDA has received 12 reports of bone marrow hypoplasia or pancytopenia in dogs being treated with fenbendazole in this extra-label manner.

We are reaching out to provide you with information concerning these reports so you can make informed treatment decisions regarding your veterinary patients.

Drug Information

Fenbendazole was first approved for use in dogs on November 2, 1983. It is available as both an over-the-counter product (Panacur C and Safeguard Canine) and a prescription product (Panacur Granules 22.2%). It is approved for use in dogs for the following indications: the treatment and control of ascarids (Toxocara canis, Toxascaris leonina), hookworms (Ancylostoma caninum, Uncinaria stenocephala), whipworms (Trichuris vulpis), and tapeworms (Taenia pisiformis). The labeled dose for dogs is 50 mg/kg (22.7 mg/lb) of body weight for 3 consecutive days.

FDA is aware that many animal drug formularies recommend alternate dosing regimens for the treatment of a variety of other parasite species not included in the labeled indications. Some of these dosing regimens recommend doses greater than 50 mg/kg and treatment times longer than 3 days.

Cases

Twelve reported canine cases of bone marrow hypoplasia or pancytopenia received fenbendazole for longer than the labeled duration of 3 days. Treatment length ranged from 5 to 14 days. Treatment was prescribed for a variety of parasitic conditions, including Giardia, flukes, coccidia, lungworms, and skin parasites. Eight of the twelve cases were more strongly associated with administration of fenbendazole because clinical signs developed soon after administration of the drug. In addition, these cases were not associated with concomitant vaccinations, medications, or other comorbidities. Several of the cases were diagnosed with bone marrow hypoplasia via a bone marrow biopsy, while some of the dogs were diagnosed with pancytopenia based on bloodwork and did not undergo further diagnostics (e.g., bone marrow biopsy).

FDA has also received a few reports of instances of bone marrow hypoplasia in other species receiving other fenbendazole products. 

In addition to reports received by FDA, there are several reports in the literature of bone marrow hypoplasia associated with fenbendazole use in multiple species.¹

Fenbendazole is safe and effective when used at the labeled dose and duration and for the labeled indications. This letter is meant to apprise you of the risks associated with extra-label use of fenbendazole so you can better prepare animal owners in cases where fenbendazole may be prescribed for an extra-label use. 

FDA is committed to promoting and protecting animal health by ensuring the safety and effectiveness of approved animal drugs. For more information, please contact CVM’s Education & Outreach Staff at AskCVM@fda.hhs.gov. 

We urge veterinarians and pet owners to report side effects involving fenbendazole to Intervet Inc at 1-800-441-8272. You can also report these events directly to FDA. For additional information about reporting adverse drug experiences for animal drugs, contact FDA at 1-888-FDA-VETS or https://www.fda.gov/reportanimalae.

Sincerely, 

FDA’s Center for Veterinary Medicine

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¹Bonar, CJ, AH Lewandowski, and J Schaul, 2003, Suspected Fenbendazole Toxicosis in 2 Vulture Species (Gyps Africanus, Torgos Tracheliotus) and Marabou Storks (Leptoptilos Crumeniferus), Journal of Avian Medicine and Surgery, 17(1):16–19.

Gary, AT, ME Kerl, CE Wiedmeyer, SE Turnquist, and LA Cohn, 2004, Bone Marrow Hypoplasia Associated with Fenbendazole Administration in a Dog, Journal of the American Animal Hospital Association, 40(3):224–229. 

Howard, LL, R Papendick, IH Stalis, JL Allen, M Sutherland-Smith, JR Zuba, DL Ward, and BA Rideout, 2002, Fenbendazole and Albendazole Toxicity in Pigeons and Doves, Journal of Avian Medicine and Surgery, 16(3):203–210.

Nementh, A, O Mahony, N Kakar, and CL Fellman, Anorexia and Lethargy in a Dog with Presumed Giardiasis, Clinician’s Brief, Nov 2019:17–20.

Weber, MA, MA Miller, DL Neiffer, and SP Terrell, 2006, Presumptive Fenbendazole Toxicosis in North American Porcupines, Journal of the American Veterinary Medical Association, 228(8):1240–1242.

Weber, MA, SP Terrell, DL Neiffer, MA Miller, and BJ Mangold, 2002, Bone Marrow Hypoplasia and Intestinal Cell Necrosis Associated with Fenbendazole Administration in Five Painted Storks, Journal of the American Veterinary Medical Association, 221(3):417–419.