Theanine

Theanine (L-theanine) is a small, water-soluble amino-acid derivative used in leave-on products at low percentages (commonly ~0.1–2%) for soothing/anti-redness support. It is not an exfoliating acid, fragrance, or preservative and has a low rate of irritation in cosmetic use, though any leave-on ingredient can occasionally sting on severely compromised eczema skin. Given its generally favorable tolerability but non-zero risk in highly reactive barrier-impaired patients, it best fits a "very gentle" profile rather than inert. Safety Notes: In commercial skincare, L-theanine is most often used as a calming/anti-stress amino acid at very low levels (typically ~0.01–0.1%) in leave-on serums, moisturizers, and toners where it functions as a supportive soothing ingredient rather than a primary active. Higher-strength consumer-available formulas (especially minimalist “amino acid” or “calming” serums/ampoules) are observed around ~0.5–2.0%, above which use becomes uncommon due to solubility, cost, and diminishing practical benefit in standard cosmetic formats. Rinse-off products, when they include theanine, tend to sit at the low end because of limited contact time.