Cystine

Cystine is an amino-acid–derived conditioning/skin-support ingredient typically used at low concentrations in leave-on and rinse-off formulas, where it is generally well tolerated and not an acid/exfoliant or surfactant. Available safety/patch-test experience for amino acids suggests a low rate of irritant reactions, though compromised eczema skin can still sting with many otherwise “gentle” solutes. Given the limited irritancy signal but real-world reactivity in highly sensitized barriers, it fits best as very gentle rather than inert. Safety Notes: In commercial skincare, cystine is most often used as a low-level skin-conditioning/antioxidant-support amino acid in leave-on serums/creams and occasional rinse-off cleansers, where it commonly appears at trace-to-low active levels around 0.001–0.1%. Higher-strength consumer products (typically leave-on “amino acid/peptide” concentrates or anti-aging/repair serums) have been observed up to about 1–2% before solubility, odor/color stability, and formula clarity/feel constraints become limiting; above this is uncommon in OTC skincare and more typical of supplement or professional-use contexts.