Methylene blue is an extremely effective antidote for acquired methemoglobinemia but has important adverse effects if given in excess of recommended dose.

Below is the usual dose of methylene blue for treatment of methemoglobinemia

1-2 mg/kg of 1% solution IV with a repeat dose given if there is inadequate response to the first one

Adverse effects include:

• >4 m/kg -- Reversible skin, feces, and urine discoloration
• 5-7 mg/kg -- EKG abnormalities (T-wave inversions, diminished R-waves), shortness of breath, chest discomfort, diaphoresis, nausea, diarrhea, abdominal discomfort
• Paradoxically, between 4 and 15 mg/kg, it may cause methemoglobinemia

References

Lo, J et al. A review of methylene blue treatment for cardiovascular collapse. 2014 J Emerg Med 46(5):670-679.