Flavonoids are a diverse group of plant polyphenols found in many fruits, vegetables, and herbal remedies. Their use in the support or treatment of colds is primarily based on scientific investigation into their antiviral, anti-inflammatory, and immunomodulatory properties. Laboratory and some clinical research have shown that certain flavonoids—such as quercetin, catechins, and hesperidin—can inhibit the replication of respiratory viruses, including rhinoviruses and influenza viruses, which are common causes of the common cold. These compounds may also modulate immune cell function and reduce the production of pro-inflammatory cytokines, potentially lessening the severity and duration of cold symptoms.

However, while in vitro (test tube) and some animal studies provide promising data, human clinical trials are more limited and results are mixed. For example, some randomized controlled trials suggest that flavonoid supplementation can reduce the incidence and duration of upper respiratory tract infections, but others report minimal benefit. The variability in flavonoid types, dosages, and study populations makes it difficult to draw firm conclusions. Overall, the evidence base is growing but not yet definitive, so while scientific rationale exists, further large-scale clinical trials are needed to fully establish the effectiveness of flavonoids in treating or preventing colds.