It is found in many fruits, vegetables, leaves, seeds, and grains; capers, red onions, and kale are common foods containing appreciable amounts of it.

[6] Quercetin is one of the most abundant dietary flavonoids,[2][3] with an average daily consumption of 25–50 mg.[7] In red onions, higher concentrations of quercetin occur in the outermost rings and in the part closest to the root, the latter being the part of the plant with the highest concentration.

[11] Quercetin is the aglycone form of a number of other flavonoid glycosides, such as rutin (also known as quercetin-3-O-rutinoside) and quercitrin, found in citrus fruit, buckwheat, and onions.

[2] Quercetin forms the glycosides quercitrin and rutin together with rhamnose and rutinose, respectively.

[15] Because it undergoes rapid and extensive metabolism, the biological effects presumed from in vitro studies are unlikely to apply in vivo.

[20] Because of low solubility and poor bioavailability of quercetin, derivatives have been synthesized to overcome these challenges and enhance its biological activity, leading to compounds with improved properties for possible therapeutic applications.

[24] In contrast, quercetin did undergo extensive phase II (conjugation) to produce metabolites that are more polar than the parent substance, hence are more rapidly excreted from the body.

[18] Compared to other flavonoids, quercetin is one of the most effective inducers of the phase II detoxification enzymes.

An in vivo study found that quercetin supplementation slows the metabolism of caffeine to a statistically significant extent in a particular genetic subpopulation, but in absolute terms the effect was almost negligible.

§ 321(g)(1)(C)] because they are intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease",[36][37] conditions not met by the manufacturers.

Little research has been conducted into the safety of quercetin supplementation in humans, and the results are insufficient to give confidence that the practice is safe.