Quercetin is a bioflavonoid that is found in onions, apples, black tea, and grapefruit. Quercetin has antioxidant activity and has long been used in herbal medicine. Quercetin is typically used as a blood vessel protectant.
Quercetin has also been studied for its effect on managing allergies. Quercetin has shown to inhibit mast-cell secretions and has the ability to downregulate histamine production (Histamine Intolerance) via inhibition of the enzyme histidine decarboxylase (HDC) mRNA from human mast cells.