Methylene blue N is widely used in medical settings as a diagnostic tool and treatment agent. In clinical diagnostics, it is employed as a staining agent to identify certain bacteria, parasites, and cellular components under a microscope. It is particularly effective in highlighting live cells and tissues, making it valuable in histology and pathology. Additionally, it is used in the treatment of methemoglobinemia, a condition where the blood cannot carry oxygen effectively, by converting methemoglobin back to hemoglobin. In surgical procedures, it serves as a dye to mark tissues or structures, aiding in precision during operations. Beyond medicine, it is utilized in aquaculture to treat fungal infections in fish and their eggs. Its applications also extend to research, where it acts as a redox indicator in biochemical experiments.