Despite being widely used as a dewormer in many species, toxicity has been reported. Birds (storks, pink pelicans, vultures, pigeons and doves) and reptiles (vipers, turtles and tortoises) have shown toxicity associated with bone marrow suppression, intestinal crypt cell necrosis and distal villi sloughing. Abortions in domestic ruminants have been associated with concurrent use of anti-trematode therapeutic agents.
Fenbendazole is poorly absorbed from the gastrointestinal tract in most species. The LD50 in laboratory animals exceeds 10 g/kg when administered orally.