Quercetin

Quercetin is a plant-derived flavonoid used as an antioxidant/anti-inflammatory active in topical products, typically at low concentrations (about 0.1–1%), but it has limited robust human irritancy data and can be delivered in solvent systems that increase stinging. While many users tolerate it, botanical polyphenols do produce occasional irritation or contact reactions in reactive/eczema-prone skin, so I score it as mild to reflect nonzero risk and prioritize patient safety. Safety Notes: In commercial skincare, quercetin is most often used in leave-on antioxidant/anti-pollution serums and creams at very low levels (~0.001–0.05%) due to solubility, color/odor, and oxidation/stability constraints, with some products using solubilized or encapsulated forms around 0.1–0.5%. High-strength consumer-available “flavonoid/antioxidant concentrate” serums and booster drops occasionally reach ~1–2% quercetin (typically with solvent systems, glycols, cyclodextrin/encapsulation, and strong stabilization), while rinse-off products generally sit at the lower end because of limited contact time and cost.