Flavonoid supplements are concentrated extracts of certain plants. How Stuff Works says that flavonoids are also known as phytochemicals, which are found in some plants. Berries are particularly rich in flavonoids. Some flavonoids may help protect against cancer and heart disease, according to How Stuff Works.
However, News-Medical.net reports that some flavonoids may promote the growth of cancer and affect the endocrine system. News-Medical.net reports that rather than being thought of as healthy, flavonoids should be thought of as being very potent and active chemicals. They state that eating plants rich in flavonoids is probably safe, but caution should be used when taking concentrated extracts of flavonoids. Because these chemicals are thought of as nutritional supplements they are not regulated by the Food and Drug Administration.