Quercetin is a plant-derived flavonoid found in foods like apples, onions, and berries and is commonly used as a dietary supplement. It has antioxidant and anti-inflammatory properties and has demonstrated antiviral and immune-modulating effects in laboratory and small human studies. For upper respiratory infections, clinical data suggest it may modestly reduce symptom severity or duration in some individuals. It is generally well tolerated and widely accessible, making it a commonly used adjunct during viral URIs. Quercetin is often taken alongside vitamin C, which may enhance its absorption.