Quercetin is a bioflavonoid found primarily in red wine, apples, grapefruit, onions and black tea. The substance is an antioxidant and may have anti-inflammatory properties. Some people take quercetin as a supplement for treating and preventing various health conditions.
Quercetin, like other antioxidants, eliminates free radicals. These are unstable molecules in the body that can damage nearby cells, leading to aging effects, heart disease and even cancer. Quercetin also may be beneficial for treating allergies, asthma and other inflammatory conditions.
Oral quercetin supplements have been associated with headaches and tingling of the feet or hands. Heartburn or acid reflux may occur with larger doses, but this often can be prevented by taking quercetin with food.
Side effects reported with the injectable form of quercetin include flushing and sweating, shortness of breath, nausea and vomiting.
Rare cases of kidney damage have been reported after high doses of injected quercetin. Also, although very uncommon, some people have an allergic reaction to these supplements.
Quercetin may lessen the effectiveness of fluoroquinolone antibiotics. Additionally, quercetin is a phytoestrogen with similar functions to the estrogen hormone, and may interact with hormone replacement therapy or other estrogen supplementation.