Identity
Methylene blue (methylthioninium chloride, colour index CI 52015, "Basic Blue 9") is not a peptide but a small-molecule phenothiazine dye — one of the oldest synthetic drugs, in medical use since the late 19th century. It is worth placing in the register precisely because it is sold across research, biohacker and emerging cosmetic channels under one name while meaning three different things.
The approved reality
As a licensed medicine, methylene blue is a first-line intravenous treatment for methemoglobinemia — a condition where hemoglobin can't release oxygen properly. This use is decades-established and genuinely grade A. But it is an emergency, intravenous, physician-administered drug use — and it is not what a cosmetic or "longevity" buyer is reaching for. Everything below stands on much thinner ground.
The cosmetic newcomer
The skin-care interest traces mainly to in-vitro work on cultured human fibroblasts and reconstructed-skin models, where low-concentration methylene blue acted as an antioxidant, lowered reactive oxygen species and delayed markers of cellular senescence. That is a legitimate and interesting signal — but it is cell-culture and tissue-model data, not controlled human topical trials, so the honest grade for "anti-aging skincare" is early-stage. Two practical facts also matter: methylene blue is an intense dye that stains skin, nails and fabric, a real formulation obstacle, and "antioxidant in a dish" has a long history of not translating to visible human benefit.
The mechanism, and the catch
At low doses methylene blue can act as an alternative mitochondrial electron carrier, which is the basis of the "mitochondrial / nootropic / longevity" marketing. That mechanism is real in principle, but the human evidence for healthy-user cognitive or longevity benefit is weak, and the safety framing is often dishonestly gentle. Methylene blue is a potent monoamine-oxidase inhibitor at relevant doses — combined with serotonergic drugs it can trigger serotonin syndrome — and it is contraindicated in G6PD deficiency. A molecule this old and this blue reads as harmless; the interaction profile says otherwise.