Isostearic Acid

Isostearic acid is a fatty acid primarily used as an emollient/surfactant component and viscosity modifier, typically at low-to-moderate concentrations in creams, makeup, and cleansers. Human patch-test data and broad cosmetic use suggest it is generally well tolerated, but as a lipophilic fatty acid it can still sting or irritate compromised or eczematous skin (especially in leave-on products or when combined with other irritants). Given this low but nonzero reactivity risk in highly sensitive populations, a gentle (not inert) score is most consistent with patient safety. Safety Notes: In commercial skincare and color-cosmetic products, isostearic acid is often used at very low levels (~0.05–0.3%) as an emulsifier co-aid, pigment wetting/dispersing agent, or mild viscosity/feel modifier in both leave-on and rinse-off systems. Most leave-on creams/lotions and sunscreens commonly fall around ~0.5–5% where it functions as a structuring lipid and emollient and supports W/O emulsions and pigment dispersion. The highest OTC consumer-available levels are seen in anhydrous balm/stick formats and some makeup bases where it can act as a primary fatty-acid/emollient component, reaching ~15–30% without being prescription- or professional-only; there is no specific EU/FDA maximum limit for isostearic acid itself beyond general cosmetic safety and labeling requirements.