Phenazopyridine toxicosis in the cat

Severe illness developed after the oral administration of several drugs, including large doses of phenazopyridine (100 mg TID for 4 days) to a cat with dysuria and hematuria. Hemolysis and icterus were evident in blood serum and plasma after day 4 of drug administration, and many hemolyzed red blood cell "ghosts" containing Heinz bodies were observed on a stained blood smear. The cat became anemic and died within 48 hours after the last dose was administered. In an attempt to confirm a cause-and-effect relationship between drug administration and disease, 100 mg of phenazopyridine was given TID (65 mg/kg/day) for 3 days to a clinically normal cat. Nearly 50% of the hemoglobin was oxidized to methemoglobin during the course of phenazopyridine administration. Lower dosages of phenazopyridine (10 and 20 mg/kg/day) for longer periods of administration to 2 other clinically normal cats did not result in illness or anemia; however, the number and size of Heinz bodies and blood methemoglobin content were increased. Evidence of hepatic injury was observed in the clinically affected cat and in 2 of the experimental cats. The relationship between hepatic injury and toxic signs was not determined. Combination products recommeneded for treatment of cystitis in man often contain phenazopyridine. Such products should be avoided in cats unless a safe, effective dosage for phenazopyridine can be established.