A multivitamin is any preparation containing more than a single vitamin. Tablets and injectable forms (for example Vitaped and Hospira) are available. In common usage, "multivitamin" refers to tablets that contain various vitamins usually along with dietary minerals and, occasionally, herbal extracts.
By supplementing the diet with additional vitamins and minerals, multivitamins can be a valuable tool for those with dietary imbalances or different nutritional needs . People with dietary imbalances may include those on restrictive diets and those who can't or won't eat a nutritious diet. Pregnant women and elderly adults have different nutritional needs from other adults, and a multivitamin may be indicated by their physician.
Orthomolecular medicine proponents generally recommend individually optimized, often higher, vitamin intakes. They recommend more absorbable forms of vitamins and minerals, in inexpensive but higher potency formulas, spread across the day. Often iron-free formulas, sometimes copper-free formulas are preferred.
Multivitamins help bridge the nutritional gaps found in most people's diets. According to an article published in 2002 in the Journal of the American Medical Association, Harvard researchers David H. Fletcher, M.D., MSC and Kathleen M. Fairfield, M.D., DrPH reported the following: "Most people do not consume an optimal amount of all vitamins by diet alone. It appears prudent for all adults to take vitamin supplements." 
While multivitamins can be a valuable tool to correct dietary imbalances, it is worth exercising basic caution before taking multivitamins, especially if any medical conditions exist. Pregnant women should generally consult their doctor before taking any multivitamins. Severe vitamin and mineral deficiencies require medical treatment and cannot be treated with common over-the-counter multivitamins. Special vitamin or mineral forms with much higher potency are typically available as individual components, specialized formulations, or available by prescription.
Multivitamins may be dangerous if taken in large amounts, due to the toxicity of certain components, principally iron. In particular, other components at extraordinarily levels in high potency forms include (but are not limited to) vitamin A, vitamin D, vitamin B6, time release niacin (especially old versions over one hour), and potassium. Total iron content of the whole bottle is the primary concern for child safety. There also are strict limits on the retinol content for vitamin A during pregnancies that are specifically addressed by prenatal formulas. Additionally, various medical conditions and medications may adversely interact with multivitamins.
For normal adults taking a multivitamin for general health purposes, conventional medicine and government authorities recommend that a multivitamin should contain 100% DRI or less for each ingredient. However, many common brand supplements in the US use 100–200% of the DRI for certain vitamins or minerals. Higher potency versions of multivitamins, sometimes labeled as megavitamins, are available in the US. Many brands offer low iron or iron-free versions of their multivitamins.
Recent research published in the Journal of the National Cancer Institute suggests that taking multivitamins more than seven times a week can increase the risk of advanced and fatal prostate cancer. Some analyses have suggested that beta-carotene, vitamin A and vitamin E supplements may shorten life rather than extend it.
Regulations by governmental agencies
The United States of America
Because of their categorization as a dietary supplement by the Food and Drug Administration (FDA), most multivitamins sold in the U.S. are not required to undergo the same rigorous testing procedures typical of pharmaceutical drugs.
However, some multivitamins contain very high doses of one or several vitamins or minerals and therefore require a prescription in the U.S. Since such drugs contain no new substances, they do not require the same testing as would be required by a New Drug Application, but were allowed on the market as drugs due to the FDA's Drug Efficacy Study Implementation program. See 36 Fed. Reg. 6843 (Apr. 9, 1971).
Multivitamin product components
Many basic commercial products selling multivitamin supplements usually contain the following ingredients: vitamin C, B1, B2, B3, B6, folic acid (B9),B12, B5(pantothenate), H (biotin), A, E, D3, K1, potassium iodide, cupric (sulfate anhydrous, picolinate, sulfate monohydrate, trioxide), selenomethionine, borax, zinc, calcium, magnesium, chromium, manganese, molybdenum, betacarotene, and iron. Other formulas may include additional ingredients such as other carotenes (e.g. lutein, lycopene), higher than RDA amounts of B, C or E vitamins, "near" B vitamins (inositol, choline, PAPA), betaine hydrochloride, lecithin, citrus bioflavinoids or forms variously described as more absorbable.
- Dietary supplements: Using vitamin and mineral supplements wisely, Mayo Clinic
- Fletcher RH & Fairfield KM. (2002) Vitamins for Chronic Disease Prevention in Adults, JAMA 287(23):3127–9
- Multivitamin prostate warning, BBC News