Quercetin is a flavonoid with antioxidant properties. This compound has many natural sources, including fruits like apples and grapefruits, certain types of nuts, green tea; it is one of the antioxidants associated with good health effects of red wine. Flavonoids like quercetin are generally considered to be very healthy, and supplements are often recommended to enhance basic health. In addition, flavonoids are water-soluble, so the body usually excretes them if they are taken in excess. Despite these positive effects, however, serious quercetin side effects have been reported when very high levels are consumed.
Supplementation with quercetin is sometimes recommended for conditions like allergies, diabetes, vision problems caused by cataracts or retinal issues, atherosclerosis, and may also be recommended for anti-aging regimens. The quantity of quercetin varies in different supplements, but anywhere from 500-1,500 mg per day is a typical dosage for health benefits. On the other hand, some health professionals feel that even these levels of quercetin are too high and put a person at risk for quercetin side effects. These professionals suggest that quercetin should be obtained through a healthy diet rather than supplementation.
The most common of the quercetin side effects appears to be heartburn that occurs soon after taking a large dose. Acid reflux may also occur. Taking the supplement with or shortly following a meal will usually get rid of this relatively mild side effect.
Other quercetin side effects may include an increase in inflammation in the body. Although antioxidants are best known for their ability to fight inflammation processes in the body, a very large dose can actually have the opposite effect and induce inflammation. Reports on this side effect indicate that this often shows up as pain and stiffness in the joints.
Another possible side effect involves abnormal hormone levels. Estrogen, a female hormone, appears to be particularly susceptible to high quercetin levels. One study showed much higher estrogen blood levels in post-menopausal women taking estrogen therapy in addition to very high levels of quercetin when compared to a similar group of women who did not supplement with quercetin. High levels of female hormones have been associated with a higher risk of reproductive cancers and breast cancer in women.
Some scientists believe that quercetin side effects may also include a higher risk of certain cancers. While normal levels of quercetin from a healthy diet appear to help fight many cancers, very high levels as obtained from over-supplementing might have the opposite effect. This appears to be because high concentrations of quercetin can actually cause DNA damage.
As quercetin may potentially cause genetic damage, many health professionals recommend that pregnant women avoid strong supplements. This is to make sure genetic changes do not occur in the developing baby. Finally, very high doses should also be avoided in children because there is a slight association between a high concentration of quercetin in the body and certain leukemias in children.