Dietary supplements are products designed to augment your daily intake of nutrients, usually the vitamins and minerals. Other substances also fall into this category: botanical (herbal) products, amino acids, essential fatty acids and oils, enzymes, probiotics all sold as dietary supplements.
In most cases, multiple-vitamin supplements provide all the basic micronutrients (vitamins and minerals) your body needs. These multiple vitamins are generally safe because they contain only small amounts of the each nutrient.
Individual nutrients can also be sold as dietary supplements, but usually in larger amounts than what’s found in a typical multiple-vitamin. They may be used to treat a deficiency, such as iron deficiency, but sometimes they’re used therapeutically to treat specific health conditions or risk factors.
For example, large doses of niacin may be used to raise good cholesterol, and folic acid has been used to reduce the risk of a birth defect called spina bifida.
Scientific research supports some of the benefits of using many dietary supplements for certain health conditions, but in many more cases, the effectiveness has not been backed up by the research evidence. The National Institutes of Health Office of Dietary Supplements has dietary supplement fact sheets that assess the evidence for (and against) the therapeutic use of a large number of dietary supplements.