Dose-Dependent Effects, Metabolic Paradoxes, and Nutritional Implications of Flavonoids and Cancer

Flavonoids are widely consumed dietary polyphenols that have been extensively investigated for their antioxidant, anti-inflammatory, and anticancer properties. However, their role in oncology is more complex than is often assumed, as their biological effects may vary according to dose, bioavailability, metabolic transformation, tumour type, disease stage, and treatment context. This narrative review examines the dual and context-dependent nature of flavonoids in cancer biology, with emphasis on redox-dependent liabilities, genotoxicity, pharmacokinetic limitations, gut microbiota-mediated variability, and herb-drug interactions. The reviewed data indicate that flavonoids may exert protective, chemosensitizing, or regulatory effects under some conditions, while under others they may promote adaptive antioxidant responses, alter drug disposition, influence immune signaling, or produce adverse outcomes at supraphysiological exposure levels. Current evidence therefore does not support a uniformly beneficial interpretation of flavonoids in oncology. Rather, their significance lies in their biological duality and exposure-dependent behavior.