The use of complementary therapies (CTs) including the ingestion of natural health products is common amongst cancer patients. Of concern to clinicians and patients alike is the possibility that concurrent use of CTs may interact poorly with standard biomedical therapy, especially chemotherapy and radiotherapy. Proponents of CTs argue that taking these therapies can help reduce the side effects of conventional therapy as well as provide an additional anticancer effect. Opponents, however, insist that the potential for harm is too great to warrant the risk of concurrent administration. There are promising examples of how specific natural health products could provide patient benefit even when given in close proximity to both chemotherapy and radiotherapy. Unfortunately, however, due in part to a rather limited evidence base, caution is warranted when considering the issue of therapeutic interactions. Avoidance of high dose antioxidants concurrent to both chemotherapy and radiation therapy is still probably the wisest course of action.