No consistent side effects have been linked to the flavonoids except for catechin, which can occasionally cause fever, anaemia from breakdown of red blood cells, and hives.^{85, 86} These side effects subsided when treatment was discontinued.

In 1980, quercetin was reported to induce cancer in animals.⁸⁷ Most further research did not find this to be true, however.^{88, 89} While quercetin is mutagenic in test tube studies, it does not appear to be mutagenic in animal studies.⁹⁰ In fact, quercetin has been found to inhibit both tumour promoters⁹¹ and human cancer cells.⁹² People who eat high levels of flavonoids have been found to have an overall lower risk of getting a wide variety of cancers,⁹³ though preliminary human research studying only foods high in quercetin has found no relation to cancer risk one way or the other.⁹⁴ 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.⁹⁵