Dapsone is among the medicine that belongs to sulfone groups and being used to treat leprosy and dermatitis. It inhibits the synthesis of dihydrofolic acid by competing with para-aminobenzoic acid. This is a case of a lady in her mid70’s known case of pyoderma gangrenosum recently started on dapsonepresented to ED with complaint of shortness of breath, discoloration of fingers for 4days. Patient had saturation of 88% room air. Relevant examination showed blue discoloration of the finger pulps. Arterial blood gas analysis showed oxygen saturation of 98%RA and PaO2 of 87mmHg, suggestive of positive saturation gap. She was treated with IV methylene blue on the impression of methemoglobinemia and her discoloration improved rapidly. An approach to hypoxia in ED must be considered in context of saturation gap in patients suspected of having toxicological cause. Methemoglobinemia treatment requires methylene blue administration that causes rapid resolution of symptoms.

Keywords: Dapsone, Methemoglobinemia, Methylene blue, Saturation gap.