Radiation and some chemotherapeutic agents used in conventional cancer treatment generate reactive oxygen species (ROS), and a high ROS level diminishes cellular antioxidant capacity and leads to apoptosis and cancer cell death. Antioxidant supplements are consumed widely by cancer patients in order to prevent toxic side effects of cancer treatment to normal tissues and organs. However, the effects of antioxidant supplementation in cancer therapy were largely disappointing. There is still no consensus on the efficacy and safety of dietary antioxidant supplementation during conventional cancer therapy. In some studies, antioxidant supplements did not reduce the risk for cancer or prevent tumour growth; at the contrary, these interventions resulted in some cases to be harmful to the patients. Therefore, a guidance on antioxidant supplementation based on large clinical trials is urgently needed in order to obtain the best possible care and to avoid risky treatments for cancer patients.