Quercetin as an Adjuvant in Cancer Therapy: Biological Mechanisms, Oral Delivery Systems, and Clinical Perspectives.

INTRODUCTION: Cancer remains a major global health challenge, and quercetin, a widely distributed dietary flavonoid, has attracted increasing attention for its anti-inflammatory, antioxidant, and neuroprotective properties in oncology research. METHODS: This narrative review synthesizes recent preclinical and clinical evidence on quercetin for cancer prevention and treatment, with an emphasis on its adjuvant role, toxicity-modulating effects, and formulation strategies designed to improve delivery. RESULTS: Quercetin has been reported to enhance the activity of several chemotherapeutic agents by modulating apoptosis, drug-resistance mechanisms, and key signaling pathways; moreover, it may reduce treatment-related nephrotoxicity, cardiotoxicity, reproductive toxicity, and neurotoxicity. Nevertheless, clinical translation remains limited by poor aqueous solubility, reduced stability, rapid metabolism, and low bioavailability. In addition, clinical trial data remain sparse, and many completed studies have not yet reported outcomes. DISCUSSION: Consequently, advances in delivery systems, including nanoformulations, liposomes, and micelles, may improve quercetin stability, absorption, and tumor targeting. CONCLUSION: Overall, quercetin appears to be a promising adjunct in oncology; however, stronger clinical evidence and optimized delivery strategies are required to define its therapeutic value.