Casein

Casein is a milk-derived protein used as a film-former/conditioning agent in cosmetics, typically at low concentrations, and it is not intrinsically caustic like acids or retinoids. However, as a high–molecular weight animal protein, it has documented potential to trigger reactions in a subset of users with milk protein allergy or highly compromised eczema skin via contact urticaria or eczematous flares, even when overall population irritation rates are low. Given this sensitization/allergy risk in sensitive populations, I score it as mild rather than gentle for patient safety. Safety Notes: In modern commercial skincare, casein (a milk-derived protein) appears infrequently and typically as a minor film-forming/skin-conditioning additive in “milk protein” themed lotions, creams, and masks at trace-to-low levels (~0.01–0.5%). The highest consumer-available uses are seen in specialty rinse-off masks/cleansers and some leave-on creams marketed around milk proteins where the total milk-protein complex can reach a few percent; casein itself is typically formulated up to about 5% before odor, allergen labeling considerations, and solubility/texture constraints become limiting. No specific FDA/EU cosmetic maximum is set for casein, so the practical upper end is driven mainly by stability and sensorial limits rather than regulation.