Antioxidants have had a checkered history concerning their reported ability to prevent or treat cancer. Early studies that showed ascorbate had benefit in cancer were followed by more definitive studies that demonstrated no benefit. Recent work suggests that biological context may be key to predicting whether antioxidants impede or even promote tumorigenesis. In a recent report, the antioxidants N-acetylcysteine and vitamin E accelerated tumorigenesis of lung cancer in mice. antioxidants decrease reactive oxygen species (ROS) levels, which paradoxically increase the proliferation rate of lung cancer cells, resulting in greater tumor burdens and reduced survival. Increased proliferation rates result from decreased expression of the genomic watchdog protein p53. In mice lacking p53, neither anti-oxidant affects tumor growth. But antioxidants can be used to kill cancer, at least in rodents. High concentrations of the "antioxidant" ascorbate, achievable only by injection in vivo, result in the production of ascorbate radicals and hydrogen peroxide in the extracellular fluid that kills cancer cells, but not normal cells. In preliminary human trials, ascorbate reduced the toxicity of chemotherapy, but showed no statistical benefit on disease progression. Vitamin C is beneficial when it acts as an oxidant. These studies are consistent with others that suggest that even tumor suppressor genes, such as Nrf2, which stimulate innate cellular stress protection pathways that reduce ROS, can promote cancer progression. Nrf2 is required for the cancer preventive effects of compounds such as sulforaphane, but Nrf2 can help maintain an aggressive tumor phenotype by stimulating proliferation and offering protection from chemotherapy. Context determines whether a specific gene is a tumor enhancer or a suppressor. Such paradoxical behavior creates difficult problems for the development of conventional therapeutics to fight cancer. Personal genomic analysis may provide the means to identify context to avoid these paradoxes and obtain a successful outcome. However, cancer prevention may be more difficult than previously thought.