Methylene blue (CI 52015), also known as methylthioninium chloride, has many uses in biology and chemistry; for example, it can be used as a stain and as a medication. Methylene blue should not be confused with methyl blue, another histology stain, new methylene blue, nor with the methyl violets often used as pH indicators.
Methylene blue is a heterocyclic aromatic chemical compound (a phenothiazine derivative) with the chemical formula C16H18N3SCl. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in water. The hydrated form has 3 molecules of water per unit of methylene blue. Methylene blue has a pH of 3 in water(10g/l) at 25 °C (77 °F).
Methylene blue was first prepared as a stain in 1876 by German chemist Heinrich Caro.[2] It was discovered to be an antidote to carbon monoxide poisoning and cyanide poisoning in 1932 by Matilda Moldenhauer Brooks.[3]
It is on the World Health Organization's List of Essential Medicines[why?].[4]