Abstract Patients with cancer are taking antioxidant supplements in combination with conventional chemotherapy and radiotherapy to enhance the anticancer activity and to reduce the side effects of conventional treatment. Many oncologists are concerned that the protective mechanisms of antioxidants may not distinguish between normal and malignant cells, so that supplements may interfere with the anticancer activity of the conventional therapies. Not all antioxidant supplements are equivalent; antioxidants counteract free radical activity through multiple mechanisms of action.