Lysine

Lysine is a basic amino acid used in skincare as a humectant/skin-conditioning agent, typically at low concentrations, and it is generally well tolerated even on sensitive skin. As an endogenous building block, it has low inherent irritancy and patch-test reactions are uncommon, though stinging/irritation can occur in highly compromised barriers or when the overall formula pH is not skin-friendly. Given rare but possible reactivity in eczema-prone patients, it is best classified as very gentle rather than inert. Safety Notes: In commercial skincare, lysine is most often used as an amino-acid/NMF-supporting component or as part of an amino-acid blend, where it commonly appears at very low individual levels (about 0.01–0.3%) in leave-on moisturizers, serums, and some rinse-off cleansers. Higher-strength consumer products (e.g., barrier-repair creams, “amino acid complex” concentrates, and some exfoliating/actives formulas using lysine as a buffering/skin-conditioning amino acid) can reach multi-percent levels, with the upper end around ~5% in leave-on products before solubility, pH, sensory tackiness, and formula stability become limiting. No specific EU/FDA cosmetic maximum is set for lysine, so observed use levels are primarily constrained by formulation performance rather than regulation.