Azure B is a cationic dye and the major metabolite of Methylene blue. Azure B is used in making Azure eosin stains for blood smear staining. Azure B is a high-potency, selective and reversible inhibitor of monoamine oxidases (MAO)-A, with IC50s of 11 and 968 nM for recombinant human MAO-A and MAO-B, respectively. Azure B possesses significant antidepressant-like effects.