BACKGROUND: Drug-induced methaemoglobinaemia is a rare but relevant condition with distinct features. CASE PRESENTATION: A woman in her sixties developed syncope and hypoxia following an extensive dental procedure. In the emergency department, she was stable but had dark arterial blood and oxygen saturation (SpO2) of 88 %, refractory to oxygen therapy. Blood gas analysis confirmed a methaemoglobin level of 25 % (normal <
1 %), attributed to the administration of 378 mg of prilocaine during the dental procedure (recommended maximum dosage: 300 mg). Treatment with intravenous methylene blue led to rapid recovery, with reduction of methaemoglobin levels. She was discharged the following day in good health. INTERPRETATION: Methaemoglobin forms when the haemoglobin iron oxidises from its ferrous (Fe2+) to ferric (Fe3+) state, which cannot bind oxygen, often triggered by high doses of drugs like prilocaine.