Methylene Blue

Methylene blue is used to treat methemoglobinemia by chemically reducing the ferric iron in hemoglobin to ferrous iron.
Methylene blue, when injected intravenously as an antidote, is itself first reduced to leucomethylene blue, which then reduces the heme group from methemoglobin to hemoglobin.
At high doses, however, methylene blue actually induces methemoglobinemia, reversing this pathway.
Since its reduction potential is similar to that of oxygen and can be reduced by components of the electron transport chain, large doses of methylene blue are sometimes used as an antidote to potassium cyanide poisoning, a method first successfully tested in 1933 by Matilda Moldenhauer Brooks in San Francisco, although first demonstrated by Bo Sahlin of Lund University, in 1926.
Severe methemoglobinemia may be treated with methylene blue.
Another use of methylene blue is to treat ifosfamide neurotoxicity.
Methylene blue increases blood pressure in people with vasoplegic syndrome (redistributive shock), but does not improve delivery of oxygen to tissues or decrease mortality.
Methylene blue has been used in calcium channel blocker toxicity as a possible rescue therapy for distributive shock unresponsive to first line agents.
Methylene blue is a dye behaving as a redox indicator that is commonly used in the food industry to test the freshness of milk and dairy products.