Methylthioninium chloride, commonly called methylene blue, is a salt used as a dye and as a medication. As a medication, it is mainly used to treat methemoglobinemia by chemically reducing the ferric iron in hemoglobin to ferrous iron. Specifically, it is used to treat methemoglobin levels that are greater than 30% or in which there are symptoms despite oxygen therapy. It has previously been used for treating cyanide poisoning and urinary tract infections, but this use is no longer recommended.
Methylene blue is typically given by injection into a vein. Common side effects include headache and vomiting. While use during pregnancy may harm the baby, not using it in methemoglobinemia is likely more dangerous.
Methylene blue was first prepared in 1876, by Heinrich Caro. It is on the World Health Organization's List of Essential Medicines.