Quercetin dihydrate. Quercetin is a natural flavonoid that, like other flavonoids, demonstrates antioxidant activity. It is found in many plant foods – such as onions, grapefruit, broccoli and apples – as well as in plant-derived beverages like tea and red wine. Berries are also believed to be a good source of bioavailable quercetin (black currants, lingonberries and bilberries). Quercetin is sometimes used as an ingredient in multivitamin preparations and herbal remedies. The amount of quercetin absorbed through the intestine varies depending on its source. Quercetin is generally safe and well tolerated when consumed in amounts naturally found in foods 1.