Oncology patients undergoing radiation treatment and chemotherapy routinely use prescription and/or over-the counter medications either as part of pre-existing comorbid conditions or in the context of conventional treatment management. A growing amount of data suggest that commonly used pharmaceuticals possess antioxidant properties, which may also partially explain some of their therapeutic efficacy. Clinical research is continuing on how such agents interact during chemotherapy and radiation when oxidative mechanisms of action are involved. Historically, such discussions centered on the category of dietary supplements, natural health products, fruits and vegetables, along with established protectant medications. Evidence confirms that some pharmaceutical agents exhibit antioxidant properties similar to dietary supplements, protectants, and may hence hinder the efficacy of chemotherapy and radiation treatment. Awareness by both healthcare providers and patients in this area is often lacking. After reviewing some of the more common and well-established pharmaceuticals, which include those prescribed during cancer treatment, caution needs to be advised especially in regards to the use of corticosteroids, as long-term randomized outcome studies ensuring safety in this area are still outstanding.