No consistent side effects have been linked to the flavonoids except for catechin, which can occasionally cause fever, anemia from breakdown of red blood cells, and hives.96, 97 These side effects subsided when treatment was discontinued.
In 1980, quercetin was reported to induce cancer in animals.98 Most further research did not find this to be true, however.99, 100 While quercetin is mutagenic in test tube studies, it does not appear to be mutagenic in animal studies.101 In fact, quercetin has been found to inhibit both tumor promoters102 and human cancer cells.103 People who eat high levels of flavonoids have been found to have an overall lower risk of getting a wide variety of cancers,104 though preliminary human research studying only foods high in quercetin has found no relation to cancer risk one way or the other.105 Despite the confusion, in recent years experts have shifted their view of quercetin from concerns that it might cause cancer in test tube studies to guarded hope that quercetin has anticancer effects in humans.106
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The information presented in Aisle7 is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. Self-treatment is not recommended for life-threatening conditions that require medical treatment under a doctor's care. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires June 2015.