What is the reversal therapy and dose for methemoglobinemia?

• A. N-acetylcysteine 140 mg/kg IV.
• B. Methylene blue 1–2 mg/kg IV.
• C. Hydroxyurea 2 mg/kg IV.
• D. Vitamin C 1 g IV.

Answer: (B)

Reversal of methemoglobinemia is achieved with methylene blue because it serves as an artificial electron donor that accelerates the reduction of ferric iron (Fe3+) in methemoglobin back to ferrous iron (Fe2+), restoring hemoglobin’s ability to carry oxygen. The usual dose is 1–2 mg/kg given intravenously over about 5 minutes. If symptoms persist after about an hour, a second 1–2 mg/kg dose can be given, but the total should generally not exceed 7 mg/kg to avoid toxicity. Be mindful that methylene blue can cause hemolysis in G6PD deficiency, so use with caution or consider alternatives in those patients. Other options listed are not standard rapid reversal therapies for acute methemoglobinemia, though vitamin C may be used as an adjunct in certain situations.