Is it permissible to take high-dose vitamin C or antioxidants during chemotherapy/radiation therapy, or does this impair the efficacy of the treatment?
Routine, unauthorized administration of high-dose vitamin C or other antioxidants during chemotherapy and radiation therapy is not recommended, as there is a theoretical and biological risk that they may neutralize the reactive oxygen species generated by these treatments, thereby potentially reducing the efficacy of the oncological therapy. Although the literature investigates the administration of intravenous, high-dose vitamin C—which exerts a pro-oxidant effect at pharmacological doses and theoretically may provide selective cytotoxicity or organoprotective effects (such as mitigating doxorubicin-induced cardiotoxicity)—clinical evidence remains non-consensual, and large phase III trials confirming efficacy are lacking. The pharmacokinetics of oral and intravenous administration differ sharply, and the clinical benefit of oral supraphysiological doses is uncertain; therefore, any antioxidant or vitamin C supplementation must occur exclusively in consultation with the treating physician, based on individual evaluation.
The primary antitumor mechanism of numerous chemotherapeutic agents and radiation therapy is the induction of intracellular oxidative stress and reactive oxygen species; traditional antioxidants may inhibit this process. In contrast, intravenous vitamin C reaching very high blood levels can paradoxically act not as an antioxidant, but as a pro-oxidant, which, according to certain preclinical and early clinical data, may enhance cytotoxicity or reduce specific organ toxicities; however, outcomes of human trials are inconsistent, so international guidelines advise caution.