Side effects of quercetin include tingling of the legs and arms as well as headache, and very high doses may result in kidney damage, according to WebMD. Patients who receive quercetin intravenously may experience additional side effects.
Quercetin may be safe when taken for 12 weeks in amounts of 500 milligrams twice per day, as stated by WebMD. The risks of larger amounts and long-term use are still unknown as of 2015. Quercetin may also be safe when patients receive it intravenously in amounts less than 722 milligrams. Patients who are pregnant or breastfeeding should avoid quercetin until additional research can verify its safety. The supplement may interact with other medications, such as certain antibiotics, cyclosporin and drugs that the liver changes, so patients should consult a doctor before taking quercetin if they are taking any other medications. Quercetin does not seem to interact with herbal remedies and supplements.
The plant flavonoid quercetin occurs naturally in foods such as apples, onions, green tea and red wine, and it may have an anti-inflammatory effect, as confirmed by WebMD. The substance may treat prostate pain and swelling, but there is insufficient evidence to support its effectiveness in treating high blood pressure, high cholesterol, heart disease and exercise-induced respiratory infections.