Leucine

Leucine is an amino acid used mainly as a skin-conditioning/humectant-support ingredient, typically at low percentages in moisturizers and barrier-repair formulas. Amino acids like leucine are generally well tolerated in patch testing with low irritancy and low sensitization potential, but compromised eczema skin can still react to otherwise “gentle” solutes or to the overall formula context. Given real-world use on highly reactive skin, I rate it as very gentle rather than fully inert. Safety Notes: In commercial skincare, leucine (an amino acid/NMF-supporting component) is most often used at very low levels (around 0.001–0.05%) as part of broader amino-acid blends in leave-on moisturizers/serums and in rinse-off cleansers where it functions as a minor skin-conditioning component. Higher-strength consumer products such as “amino acid/NMF” concentrates and some barrier-repair creams use individual amino acids at roughly 0.2–1%, with the highest observed OTC formulations reaching about 2% leucine before solubility/clarity and sensory constraints typically limit further increases (especially in leave-on aqueous systems).