Dapsone-Induced Methemoglobinemia in a Patient With Multiple Myeloma Following Autologous Stem Cell Transplantation: A Case of Conservative Management.

Dapsone is an alternative medication for the prophylaxis of Pneumocystis jirovecii pneumonia (PJP) in immunocompromised patients. One potential complication associated with dapsone use is acquired methemoglobinemia. However, cases of methemoglobinemia in patients with IgG kappa myeloma following autologous stem cell transplant are rare. We describe the case of a 67-year-old woman with IgG kappa myeloma who underwent a stem cell transplant and was participating in the RADAR Myeloma XV trial. She received dapsone for PJP prophylaxis due to an allergy to trimethoprim-sulfamethoxazole (TMP-SMX). Two weeks after starting dapsone, she presented with shortness of breath and an oxygen saturation level of 89% on room air. Her arterial blood gas analysis revealed a methemoglobin level of 10.9%. Conservative management, involving the discontinuation of dapsone and supplemental oxygen, led to clinical improvement. This case highlights the importance of maintaining vigilance for acquired methemoglobinemia, particularly in immunocompromised patients presenting with unexplained hypoxia. Prompt recognition and discontinuation of dapsone are essential for effective management. In this instance, conservative treatment proved successful at a methemoglobin level of 10.9%. This case contributes to the limited evidence supporting the efficacy of conservative management in dapsone-induced methemoglobinemia among post-transplant patients.