What is the standard dosing for methylene blue in methemoglobinemia?

• A. 1–2 mg/kg IV over several minutes; may repeat if needed.
• B. 0.5 mg/kg IV.
• C. 10 mg/kg IV every 6 hours.
• D. 15 mg/kg IM.

Answer: (A)

Methylene blue works by helping the body's NADPH-dependent system quickly reduce methemoglobin back to hemoglobin. The standard initial dosing is 1–2 mg/kg given IV over about 5 minutes. If there’s not sufficient improvement after about an hour, a second identical dose can be given. This approach balances efficacy with safety, since higher or repeated large doses increase the risk of adverse effects and toxicity (including hemolysis in G6PD deficiency and other complications). The other options are not correct because they involve too low a dose, an incorrect dosing interval, or an inappropriate route/dose for methemoglobinemia. Keep in mind methylene blue should be avoided in known G6PD deficiency and used with caution, with supportive care as needed.