Multivitamin
Summary
Daily multivitamin-mineral supplements. In the largest randomized trials (Physicians' Health Study II and COSMOS), daily multivitamins did not reduce cardiovascular events, cancer, or mortality compared with placebo in generally well-nourished adults.
Mechanism
A daily multivitamin is designed to fill gaps between dietary intake and recommended micronutrient levels. There is no established mechanism by which adding vitamins and minerals beyond adequacy would prevent cardiovascular disease or cancer, which is consistent with the null results: benefits would be expected mainly where true deficiency exists.
Reported effects
Ratings are relative to peers in the same category, not absolute claims.
Studied dosing: research context
The large trials used one daily standard multivitamin: PHS II followed 14,641 male physicians for a median of about 11 years, and COSMOS followed 21,442 older adults for a median of about 3.6 years. These are regimens used in research, not guidance.
Research context only. This is not a recommendation.
Safety
Generally safe in the large trials, with no major safety concerns reported for standard daily multivitamins. PHS II reported a small, borderline reduction in total cancer (about 8%) but no cardiovascular or mortality benefit; it did not demonstrate harm. Excess intake of specific nutrients (e.g., preformed vitamin A, iron) is the main toxicity concern in the broader literature.
Limitations
Both large trials enrolled generally well-nourished adults (physicians; older volunteers), so results may not generalize to populations with high rates of deficiency. COSMOS follow-up was only about 3.6 years, relatively short for cancer endpoints. Neither trial tested targeted supplementation in diagnosed deficiency.