Caffeic Acid

Caffeic acid is a phenolic acid antioxidant used in low concentrations in topical products, but as an acidic, bioactive polyphenol it can sting and provoke irritant dermatitis in compromised barriers (e.g., eczema), especially in leave-on formulas and when layered with other acids/retinoids. Patch testing and clinical experience with phenolic/plant-derived acids show a non-trivial rate of irritation and occasional allergy in highly reactive individuals, so I score it as moderate and advise patch testing for sensitive skin. Safety Notes: In real-world OTC skincare, caffeic acid is most often used as a minor antioxidant/soothing adjunct in complex botanical or polyphenol blends, where it can appear at trace-to-low levels around ~0.001–0.05% in leave-on serums/creams and some rinse-off cleansers. Higher-strength consumer products (typically leave-on antioxidant/brightening serums or water-free/oil-based systems using polyphenol complexes) are observed up to about 1–2%, beyond which stability (oxidation/darkening), solubility, and irritation/sensitization risk commonly limit practical formulation. No specific FDA/EU maximum is set for caffeic acid in cosmetics, so the upper end is driven primarily by formulatability and tolerability rather than regulation.