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Key Features

  • Oxidation of hemoglobin to methemoglobin can be caused by

    • Benzocaine

    • Aniline

    • Propanil

    • Nitrites

    • Nitrogen oxide gases

    • Nitrobenzene

    • Dapsone

    • Phenazopyridine

    • Many other oxidants

  • Dapsone has a long half-life and may produce prolonged or recurrent methemoglobinemia

Clinical Findings

  • Severity of symptoms depends on the percentage of hemoglobin oxidized to methemoglobin

    • Severe poisoning is usually present when methemoglobin fractions are > 40–50%

    • Even at low levels (15–20%), victims appear cyanotic because of the chocolate-brown color of methemoglobin

  • Dizziness

  • Nausea

  • Headache

  • Dyspnea

  • Confusion

  • Seizures

  • Coma


  • PO2 results are normal on arterial blood gas determinations; conventional pulse oximetry gives inaccurate oxygen saturation measurements

  • A newer pulse oximetry device [Masimo Pulse CO-oximeter] is capable of estimating the methemoglobin level

  • Severe metabolic acidosis

  • Hemolysis may occur, especially in patients with glucose-6-phosphate dehydrogenase deficiency


  • Administer high-flow oxygen

  • Activated charcoal

    • Administer 60–100 g orally or via gastric tube, mixed in aqueous slurry for ingestions within 1 h

    • Repeat-dose may enhance dapsone elimination

  • For symptomatic patients, administer methylene blue, 1–2 mg/kg (0.1–0.2 mL/kg of 1% solution) intravenously

    • The dose may be repeated once in 15–20 min if necessary

    • Patients with hereditary methemoglobin reductase deficiency or glucose-6-phosphate dehydrogenase deficiency may not respond to methylene blue

  • In severe cases where methylene blue is not available or is not effective, exchange blood transfusion may be necessary

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