Methylene blue treats methemoglobinemia by reducing iron in hemoglobin from the ferric (Fe3+) state back to the ferrous (Fe2+) state, which allows it to bind oxygen effectively.

“In the medical world, the main use of methylene blue right now is for met hemoglobinia. This condition occurs when the iron atom at the center of your hemoglobin molecule is oxidized from the Fe2+ state to the Fe3+ state. Iron in hemoglobin can be oxidized from the ferrris to the feric state. When iron is in the Fe3+ state, it has lost an electron. The loss of an electron causes iron to go from 2+ to 3+. Loss of electrons is oxidation. Gain of electrons is reduction. Methylene blue can rescue this. When iron is in that oxidized state, that Fe3+ state within a hemoglobin molecule, it doesn't hold on to oxygen.”