Used as a redox indicator in analytical chemistry, Methylene Blue Trihydrate helps detect oxidation-reduction reactions, turning colorless when reduced and blue when oxidized. It serves as a biological stain in microscopy, enhancing contrast in cell and tissue samples, particularly in nerve, bacterial, and parasitic examinations. In medicine, it treats methemoglobinemia by converting methemoglobin back to hemoglobin. It is also applied in photodynamic therapy, where it acts as a photosensitizer to destroy pathogens under light exposure. In aquaculture and veterinary medicine, it controls fungal and parasitic infections in fish. Additionally, it finds use in dyeing textiles and as a tracer in environmental studies to monitor water flow.