Quercetin is a type of plant-based chemical, or phytochemical, known as a flavonoid (see also Phytochemicals). Good sources include apples, onions, teas, red wines, and many other foods. Quercetin is also available as a dietary supplement. Quercetin appears to have anti-inflammatory and antioxidant properties. It has been promoted as being effective against a wide variety of diseases, including cancer. While some early lab results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans.