When used as prescribed, fenbendazole For Cancer often has no negative side effects. Sometimes, especially if fenbendazole is administered at levels greater than usual, chemicals generated by the parasites that are dying might result in an allergic response. Seek emergency veterinarian assistance if your pet displays any symptoms of an allergic reaction, such as facial swelling, itching, hives, diarrhoea, convulsions, or shock. When receiving this drug, dogs or cats may have diarrhoea, vomiting, or salivation. Although effects may last longer in animals with liver or renal problems, this moderate-acting medicine should stop functioning after a few days.