Complementary therapies, notably natural health products such as herbs and vitamins, are frequently used by cancer patients receiving chemotherapy and radiation therapy. There is much controversy as to whether these natural health products should be taken during conventional cancer treatments. Supporters of this practice cite beneficial effects of the antioxidant properties, while opponents are concerned about the potential for natural health product-chemotherapy/radiation related negative interactions. This involves understanding the role and effect on metabolizing enzymes. This review will highlight the present evidence for both the beneficial and negative consequences of the use of natural health products during chemotherapy and radiation therapy.