Methemoglobinemia is a rare condition with serious consequences if not diagnosed. We report the case of a 64-year-old woman with a history of allergy to sulfa drugs and a recent diagnosis of a small vessel vasculitis (ANCA-p) who started induction therapy with corticosteroids and rituximab. Due to the need for infectious prophylaxis, and considering her history, dapsone was administered instead of cotrimoxazole after ruling out glucose-6-phosphate dehydrogenase deficiency. During the admission to the hospital for her second dose of rituximab, and while being asymptomatic, she persistently presented a pulse oximetry ≪ 90% despite the administration of O2. Therefore, the infusion was postponed to study the patient. The arterial gasometric study by direct potentiometry revealed an O2 saturation of 98%, with a saturation gap > 5%. Considering the use of dapsone, a methemoglobinemia was suspected and confirmed by co-oximetry (methemoglobinemia 9%). Dapsone was suspended and one week later, her methemoglobinemia was absent.