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This case report describes a female in her late 80s who presented with progressive dyspnoea and marked hypoxia. Notably, the hypoxia was refractory to high-flow supplemental oxygen. Her medical history included myeloperoxidase (MPO) positive vasculitis, vitamin B12 deficiency with a baseline haemoglobin of 88g/dL and well-controlled hypertension. A venous blood gas showed a MetHb of 10.2% [< 2%], which was initially overlooked. She was admitted to the ward and remained refractory to supplementary oxygen. A subsequent arterial blood gas (ABG) revealed an oxygen saturation (SaO2) of 95% and a methaemoglobin (MetHb) level of 10.7%. On review of her drug history, it was noted that she was taking 100 mg of dapsone once daily for Pneumocystis jirovecii pneumonia (PCP) prophylaxis. Dapsone was discontinued; her symptoms and hypoxia rapidly improved, and she was discharged 48 hours later.