Quercetin

Updated June 29, 2026

Quercetin is one of the most abundant dietary flavonoids, found in high concentrations in red onions, capers, apples, broccoli, and green tea. It is a polyphenol with a broad range of biological activity: it inhibits pro-inflammatory enzymes (COX-1, COX-2, LOX), suppresses the NF-κB signalling pathway that coordinates inflammatory gene expression, scavenges free radicals directly, and modulates the activity of multiple kinases involved in cellular signalling. Its breadth of mechanism is both its strength and the reason pinning its effects to a single pathway is misleading.

The most clinically interesting development in quercetin research is its emerging role as a senolytic — a compound that selectively clears senescent cells. Senescent cells are cells that have lost the ability to divide but refuse to die, accumulating with age and secreting a cocktail of inflammatory cytokines known as the SASP (senescence-associated secretory phenotype). In preclinical models and early human trials, quercetin combined with dasatinib (a cancer drug) dramatically reduces senescent cell burden in various tissues. The quercetin-only senolytic effect is more modest than the combination, but still measurable, and it operates at doses achievable through supplementation rather than pharmaceutical intervention.

Cardiovascular benefits are well documented. Quercetin supplementation lowers blood pressure in hypertensive individuals across multiple meta-analyses — the effect is consistent and appears within four to six weeks at doses of 500 to 1,000 mg per day. It reduces LDL oxidation, improves endothelial function, and lowers inflammatory markers including CRP. Athletic performance data shows improved endurance and reduced exercise-induced inflammation, with one large study in cyclists showing a 3.9 percent improvement in maximal oxygen consumption after three weeks of supplementation.

Quercetin's bioavailability is its main limitation. Standard quercetin aglycone is poorly absorbed from the gut. Quercetin glycosides (found naturally in onions) absorb somewhat better. The most bioavailable supplemental form is quercetin phytosome (quercetin bound to sunflower phospholipids) or quercetin combined with bromelain or vitamin C, both of which improve absorption considerably. Isoquercetin, a glycosylated form, also absorbs better than the aglycone and is used in some clinical trials.

Standard supplement dosing ranges from 500 to 1,000 mg per day, ideally in a bioavailable form. It is generally well tolerated with a clean safety profile in trials up to 12 weeks, which is the range most studied. Quercetin has mild oestrogenic activity in some assay systems and can inhibit certain drug-metabolising enzymes (CYP3A4, CYP2C9) at high doses — relevant context for people on medications. This is general information, not medical advice.