What is the concern with antioxidants and chemotherapy or radiation? — Name withheld on request

Patients with cancer frequently inquire about supplement use. Many commonly used supplements, such as vitamin C, coenzyme Q10 (CoQ10), resveratrol, and many others, possess antioxidant properties. One concern regarding antioxidant use by patients undergoing cancer treatment relates to how antioxidants work within the body. 

Some cancer treatments, such as radiation and the anthracyclines (eg, doxorubicin), exert their anticancer effects — at least in part — by generating reactive oxygen species, also known as free radicals. These reactive species then damage cancer cells, triggering cell death. Antioxidants can neutralize these reactive species, resulting in decreased anticancer effects.  

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This does not mean that patients should avoid all dietary sources of antioxidants, as the amount contained in many supplements greatly exceeds that which would be consumed from a normal diet. However, patients receiving therapeutic radiation or chemotherapy that generates reactive oxygen species should be advised to avoid antioxidant-containing supplements during the active phases of their cancer treatment. Because this varies depending on the treatment, it is important for patients to advise their care team of potential supplement use.