In short
Quercetin is a flavonoid found widely in fruits and vegetables (including onions, apples, and berries) that, in combination with the chemotherapy drug dasatinib, forms the most-studied senolytic drug combination, known as "D+Q."
Quercetin was identified alongside dasatinib in the 2015 Mayo Clinic/Scripps senolytic screening work because the two compounds together disable different pro-survival pathways that senescent cells rely on to resist apoptosis, an effect neither compound produces as strongly alone — dasatinib primarily targets pathways active in senescent human preadipocytes (fat-cell precursors), while quercetin has broader activity against multiple survival pathways including PI3K/AKT.
The human evidence for D+Q specifically comes from a small number of early-phase, mostly open-label pilot studies — including the 2019 Justice et al. trial in idiopathic pulmonary fibrosis patients discussed in more detail under "senolytics" — that found reduced senescence markers and modest functional improvements over short treatment courses, not a large placebo-controlled trial with hard clinical endpoints.
Outside the D+Q combination, quercetin alone is also sold as a general antioxidant and anti-inflammatory supplement, but its senolytic effect specifically has only been demonstrated in combination with dasatinib in the published research to date — quercetin taken alone as a supplement has not been shown in human trials to clear senescent cells or replicate the D+Q findings, and generic antioxidant/anti-inflammatory supplement claims for quercetin carry a much thinner human evidence base than the specific senolytic-combination research.