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There is a popular belief that multivitamin and mineral (MVM) supplements can help prevent cancer and other chronic diseases. Studies evaluating the effects of MVM supplements on cancer risk have largely been observational, with considerable methodologic limitations, and with conflicting results. We review evidence from the few available randomized, controlled trials that assessed the effects of supplements containing individual vitamins, a combination of a few select vitamins, or complete MVM supplements, with a focus on the recent Physicians' Health Study II (PHS II). PHS II is a landmark trial that followed generally healthy middle-aged and older men (mean age 64 years) who were randomized to daily MVM supplementation for a mean duration of 11 years. Men taking MVMs experienced a statistically significant 8% reduction in incidence of total cancer (hazard ratio [HR]: 0.92; 95% confidence interval [CI]: 0.86-0.998; p = 0.04). Men with a history of cancer derived an even greater benefit: cancer incidence was 27% lower with MVM supplementation versus placebo in this subgroup (HR: 0.73; 95% CI: 0.56-0.96; p = 0.02). Positive results of PHS II contrast with randomized studies of individual vitamins or small combinations of vitamins, which have largely shown a neutral effect, and in some cases, an adverse effect, on cancer risk. The results of PHS II may have a considerable public health impact, potentially translating to prevention of approximately 68 000 cancers per year if all men were to use similar supplements, and to an even greater benefit with regard to secondary prevention of cancer.