Methylene blue is a medication used to treat methemoglobinemia, a condition in which the blood loses its ability to carry oxygen throughout the body. It is also used as a dye in chromoendoscopy to identify pre-cancerous lesions in the gastrointestinal tract. Additionally, methylene blue is used as an aid to visualization/orientation in a number of medical devices, including a surgical sealant film, TissuePatch, and to identify the tract for complete excision in fistulas and pilonidal sinuses. It can also be used during gastrointestinal surgeries to test for leaks. Methylene blue is contraindicated in patients who have demonstrated hypersensitivities or anaphylaxis after past administration and for those with glucose-6-phosphate dehydrogenase deficiency due to susceptibility to experiencing hemolytic anemia.