Phenazopyridine-induced sulfhemoglobinemia

To report a case of sulfhemoglobinemia in a patient receiving phenazopyridine for a urinary tract infection.

A 63-year-old white woman presented to the emergency department with complaints of fatigue and bluish discoloration of her body that had gradually progressed over the previous 6-8 weeks. About 4 months prior to presenting to the emergency department, she had started taking phenazopyridine, an over-the-counter medication for symptoms of dysuria. Because the cyanosis did not improve after the patient received oxygen and methylene blue, sulfhemoglobinemia was suspected and confirmed by spectrophotometer analysis.

Sulfhemoglobin is a green-pigmented molecule containing a sulfur atom in one or more of the porphyrin rings. It is a rare cause of cyanosis, which is usually drug induced. Sulfhemoglobinemia is suspected when a cyanotic patient has normal to near-normal oxygen tension, laboratory reports of elevated methemoglobin, and does not respond to methylene blue therapy. Sulfhemoglobinemia is relatively rare, despite the widespread use of drugs that have been reported to cause it. Predisposing factors, such as chronic constipation, present in our patient, have been suggested as a source of hydrogen sulfide.

This case of sulfhemoglobinemia, which occurred after the patient took phenazopyridine, is considered a probable adverse event according to the Naranjo probability scale.