The concept that medications could be used to prevent cancer is an attractive one, and many high-quality clinical trials support the use of such chemoprevention in defined circumstances. Aspirin has been found to reduce the risk of death from cancer.^[54] Daily use of tamoxifen or raloxifene has been demonstrated to reduce the risk of developing breast cancer in high-risk women by about 50%.^[55]Finasteride has been shown to lower the risk of prostate cancer, though it seems to mostly prevent low-grade tumors.^[56] The effect of COX-2 inhibitors such as rofecoxib and celecoxib upon the risk of colon polyps have been studied in familial adenomatous polyposis patients^[57] and in the general population.^[58][59] In both groups, there were significant reductions in colon polyp incidence, but this came at the price of increased cardiovascular toxicity.

Vitamins have not been found to be effective at preventing cancer,^[60] although low levels of vitamin D are correlated with increased cancer risk.^[61][62] Whether this relationship is causal and vitamin D supplementation is protective is yet to be determined.^[63] Beta-carotene supplementation has been found to increase slightly, but not significantly, risks of lung cancer.^[64] Folic acid supplementation has not been found effective in preventing colon cancer and may increase colon polyps.^[65]