Multivitamins and multiminerals are widely used in the United States, but their efficacy and, perhaps more importantly, the potential for harm in individuals who have cancer have received relatively little study. Beyond their general effects on health, the use of vitamins and minerals by patients with cancer has unique implications because of their potential direct effects on existing cancers, effects on factors that may influence carcinogenesis, such as immunity, and interactions with treatment. Some evidence suggests that vitamin D at higher than standard doses may improve cancer-specific and overall survival for several cancer sites. Besides vitamin D, there is little evidence that nutritional supplements lower the risk of recurrence or improve survival from cancer, although some benefits may be possible in specific subgroups. Some data suggest that higher than standard doses of some vitamins or minerals could even enhance carcinogenesis or worsen survival in patients with cancer. For example, evidence suggests that although folate supplementation administered in preneoplastic stages may lower the risk of colorectal cancer, excessive folic acid in patients with established cancer may be harmful. For prostate cancer, some preliminary evidence indicates that excess consumption of one or a combination of components in a multivitamin/multimineral may accelerate cancer progression and increase fatality. Use of aspirin is proven to lower risk of colorectal cancer, and recent evidence suggests that aspirin use in patients with colorectal cancer improves cancer-specific and overall survival, especially in patients with tumors that express cyclooxygenase-2 (COX-2). The potential beneficial or adverse effects of dietary supplements and aspirin in survivors of cancer warrant further study.