Methemoglobinemia induced by trimethoprim-sulfamethoxazole in a patient with systemic lupus erythematosus.

We herein report a case of methemoglobinemia induced by trimethoprim-sulfamethoxazole (TMP/SMX). A 41-year-old woman with systemic lupus erythematosus (SLE) received TMP/SMX for prophylaxis of pneumocystis pneumonia (PCP) on the 7th day of hospitalization. She suddenly developed dyspnea and cyanosis on the 9th day of hospitalization. The level of oxygen saturation (SaO2) decreased, and the concentration of methemoglobin (MetHb) in the blood was elevated. We diagnosed the patient with methemoglobinemia induced by TMP/SMX. Methemoglobinemia should be considered in cases of sudden dyspnea following TMP/SMX administration.