Methylene blue is a synthetic compound that has been used in many different products. It has been used as a blue dye, as a printing ink, as a photographic developer, and as a treatment for methemoglobinemia. It has also been used as an alternative to blue food coloring. Methylene blue has a variety of medical uses, including as an antimalarial drug and as an antidote for cyanide poisoning.

It is also used to treat methemoglobinemia, a condition in which the blood is unable to carry oxygen. Methylene blue is also used in the treatment of certain types of cancer.