While antioxidants from natural whole foods are beneficial for cancer patients, additional antioxidant supplements are not necessarily recommended for dogs undergoing chemotherapy or radiation therapy. Antioxidant supplements may interfere with the cancer-killing effects of chemotherapy, which works by inducing oxidative stress in cancer patients.
To understand this, we must first consider that dogs with cancer who are undergoing chemotherapy or radiation treatment are undergoing an oxidative therapy. While there is no evidence to suggest that antioxidants from whole foods should be avoided, the website oncologynutrition.org states that some studies indicate that “antioxidant supplements may interfere with chemotherapy and radiation therapy, by reducing their effectiveness. It is possible that antioxidants may protect tumor and cancer cells, in addition to healthy cells, from the oxidative damage intentionally caused by conventional cancer treatments. This, in turn, may reduce the effectiveness of the treatments.”