Methylthioninium chloride (MB)

Methylthioninium chloride, known commonly as methylene blue, is a synthetic phenothiazine dye first synthesized in 1876. It is used in a wide variety of fields, serving as a redox indicator in chemistry and as a stain in biological microscopy. Clinically, it acts as a medication used to treat methemoglobinemia—a condition where hemoglobin is oxidized to methemoglobin, which cannot carry oxygen efficiently—by reducing methemoglobin back to functional hemoglobin via NADPH-dependent pathways. Methylene blue also serves as a diagnostic tool, surgical dye, and has investigational use in neuroprotection and other disease states. While valuable as a medication and research tool, it is not a biological target but rather a drug/chemical compound acting on various cellular pathways.

Other names

Methylene blueMBBasic blue 9Swiss BlueProveblueUrelene blueSolvent blue 8Methylthionine chloride

Reduces methemoglobin to hemoglobin by acting as an electron donor (via NADPH-methemoglobin reductase pathway) Inhibits nitric oxide synthase and guanylate cyclase at high concentrations Interferes with mitochondrial electron transport, reducing production of free radicals Acts as a redox indicator, interconverting between oxidized (blue) and reduced (colorless) states