Genistein

Genistein is an isoflavone antioxidant/phytoestrogen typically used at low concentrations (often ~0.1–1%) in anti-aging/brightening formulas and is not inherently caustic like acids or retinoids. However, human tolerance data are limited and plant-derived phenolics can provoke stinging or dermatitis in reactive/eczema-prone users, especially in leave-on products and multi-active routines. Given the uncertainty and the need to err on patient safety for highly sensitive skin, I rate it as mild with occasional sensitivity possible. Safety Notes: In mass-market and prestige anti-aging leave-on products, genistein (often listed as genistein/soy isoflavones or delivered via soybean extracts) is frequently present at very low active-equivalent levels (~0.001–0.05%) as a supporting antioxidant/phytoestrogenic component. Clinically oriented cosmeceuticals and niche “isoflavone/phytoestrogen” serums and creams marketed for menopause-related skin changes and firming commonly use higher standardized genistein levels around 0.5–2%. A small number of high-strength, consumer-available specialty formulas (typically anhydrous/encapsulated leave-on serums) reach about 5% genistein; rinse-off products are uncommon and generally sit at the low end due to short contact time.