BACKGROUND: Methemoglobinemia due to the administration of sulfamethoxazole/trimethoprim has been documented in a series of case reports. However, all of these reports are on adult patients, and all patients received at least daily administration of sulfamethoxazole/trimethoprim for the treatment of active or suspected infection.

CASE REPORT: Herein we report the development of methemoglobinemia in a pediatric patient receiving sulfamethoxazole/trimethoprim three times weekly for the prophylaxis of opportunistic infections.

CONCLUSIONS: The clinician should always consider sulfamethoxazole/trimethoprim, even when administered for opportunistic infection prophylaxis at reduced doses and intervals, as a possible cause of methemoglobinemia.

Keywords: Leukemia, Myelomonocytic, Juvenile - complications, Methemoglobinemia - chemically induced, Opportunistic Infections - prevention & control, Trimethoprim, Sulfamethoxazole Drug Combination - therapeutic use