Quercetin is a plant flavonoid and natural antioxidant found in a number of foods including apples, onions, citrus fruit, red wine and berries. Several animal studies have showed that quercetin was an effective treatment for allergies because of its ability to reduce the respiratory effects of histamine.

A study published in 2007 demonstrated that quercetin reduced inflammation in the airways of rats with researchers concluding that it was a potential natural treatment for allergies.