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Methylene blue is the active substance of an authorised medicinal product and is increasingly taken as a self-administered oral preparation. It is offered to final consumers in dosed form, mostly declared as a laboratory chemical, reagent or dye rather than as a food supplement. No health-based guidance value (HBGV) for oral intake has been established in any jurisdiction. Points of departure were derived from 2-year gavage bioassays: a no-observed-adverse-effect level (NOAEL) of 1.8 mg/kg body weight per day for oxidative erythrocyte injury, and benchmark dose lower confidence limits (BMDL) of 2.6 and 10.3 mg/kg body weight per day for pancreatic islet and small intestinal neoplasms. A survey of 63 products offered to final consumers in eight European Union (EU) member states found that only five stated an intake amount. Intakes estimated from these statements, co-marketed consumer literature, user reports and documented oral therapeutic regimens spanned 6 to 200 mg per day. Margins of exposure fell from 20.9 to 0.6 against the NOAEL and from 30.2 to 0.9 against the BMDL, so that neither the required margin of 100 nor that of 10,000 was attained in any scenario, including the lowest, which is a supplier's own recommendation. No presentation examined could deliver an amount within a provisional HBGV of 0.018 mg/kg body weight per day, or 1.26 mg per day for an adult (based on NOAEL with uncertainty factor of 100 and adult bodyweight of 70 kg).