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.