Methaemoglobinaemia can be caused either by a genetic defect in red cell metabolism or haemoglobin structure, or acquired by a variety of drugs and toxins. Common drugs dapsone, nitrates, prilocaine, antimalarials, sulphonamides and dyes. Domestic causes of acquired methaemoglobinaemia include ingestion of food and water high in nitrites and nitrates exposure to aniline dyes in dyed blankets, laundry markings, freshly dyed shoes, red wax crayons and cleaning solution.

Standard pulse oximeters give spuriously low readings in the presence of excess methaemoglobin. Methylene blue is indicated in any patient with symptoms and/or signs of hypoxia (mental changes, tachycardia, dyspnoea, chest pain). It is contraindicated in G6PD deficiency. High flow oxygen should be administered.