Lamberts® quercetin is extracted from a natural plant source and each tablet provides a full 500mg of pure quercetin. Some cheap quercetin products, particularly those imported from the USA, may contain much less quercetin than the label declaration, as the weight being given refers to the plant extract, not to the quercetin content.Quercetin is found in many fruits and vegetables, with apples and onions contributing most to the typical UK diet. Each 500mg tablet provides significantly more quercetin than 1 kilo of apples. Quercetin is probably the most researched bioflavonoid as it is recognised as being more active in the body than most others.