Like many other plant chemicals, quercetin is sold as a supplement. Some lab studies suggest that quercetin acts as an antihistamine. In an allergic reaction, your body produces histamine, which causes sneezing, runny nose, and so on. Some claim that quercetin is more effective than conventional antihistamines and does not cause such side effects as drowsiness. Not a scrap of published evidence backs this idea.
Some researchers think quercetin in red wine protects the heart.
Others are interested in its cancer-preventing potential. We're a long way from knowing what it does. We think you should stick to conventional antihistamines. The new prescription brands are effective, though expensive, and they don't cause drowsiness. At least you know what you're getting.