Quercetin
Quercetin is a polyphenol flavonoid found in many plants, including red onions, broccoli, and blueberries. It is used in cosmetics primarily for its strong antioxidant and anti-inflammatory properties. It has also been studied for its ability to stabilize chemical UV filters such as avobenzone and octinoxate against photodegradation, outperforming traditional stabilizers like octocrylene and vitamin E in at least one study. It exhibits minor cytotoxicity compared to some of its polyphenol derivatives, such as rutin, indicating that higher concentrations are not necessarily better.
Quercetin acts as a potent antioxidant and has demonstrated anti-inflammatory activity both in vitro and ex vivo. It can help photostabilize certain chemical UV filters in sunscreen formulations. Research into its use as a skin-whitening agent has yielded conflicting results, with the current conclusion being that it is not effective as a cosmetic brightening ingredient.
Exhibits minor cytotoxicity compared to some polyphenol derivatives (e.g., rutin), suggesting that higher concentrations may not be safe or beneficial — more is not better with this ingredient.
