An evidence-based update on vitamins.

American adults take many types of vitamin supplements, despite limited evidence of their efficacy, especially in preventing chronic diseases such as cardiovascular disease and cancer. Supplements contain significant amounts of vitamins when consumed from multiple sources. Excess consumption of some vitamins may have detrimental health effects. Use of MMVM products appears to be safe; however, clinical outcomes have not been established. Although vitamin D and preconception folic acid may be appropriate for self care, a health care provider should monitor other vitamin supplements for disease prevention, such as niacin. Beyond supplementation as treatment for vitamin deficiencies, evidence is lacking.