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Almost one-third of American adults regularly take vitamins and supplements. If taken incorrectly or in excess, these vitamins may be a potential health hazard. Vitamins are essential nutrients which, in combination with other nutrients (e.g., fats, carbohydrates and proteins), foster normal metabolism. Vitamins also interact with each other. For example, vitamin C participates in the metabolism of folic acid, and vitamin E facilitates the absorption and storage of vitamin A. Because the biological functions of vitamins are interrelated, a diet poor in vitamins, carbohydrates, fats and proteins is not necessarily enhanced by vitamin supplementation. When vitamins are taken in excess of the Recommended Dietary Allowances or the individual's needs, the vitamins no longer function as vitamins but instead act as drugs, with such pharmacological effects as clinical toxicities and the abnormal utilization of vitamins. There are six categories that require vitamin supplements and, in some cases, megadoses. These will be discussed in detail. In addition, a brief table showing the Recommended Dietary Allowances will be given which the nurse practitioner can use in assessing nutritional needs of the client so that necessary adjustments can be made. Finally, a brief review of the potential risks and benefits of megadoses in normal, healthy adults will be given.