So, methylene blue can be used as a fabric dye; it can also be used as ink for temporary or permanent tattoos. However, medically, it’s used in the treatment of methemoglobinemia, as a urinary analgesic, anti-infective, and anti-spasmodic agent, or in endoscopy as a gastrointestinal dye, and can also be used as a rescue antidote in life threatening poisonings causing refractory shock states and other shock states.

While I sincerely doubt anyone’s state IV cancer has been cured by methylene blue, dismissing it as “a fabric dye” is wrongfully dismissive of it’s actual medical uses.