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Deficiencies of single vitamins are less often encountered than those of multiple vitamins. Although any cause of protein–calorie undernutrition can result in concurrent vitamin deficiency, most deficiencies are associated with malabsorption, alcoholism, medications, hemodialysis, total parenteral nutrition, food faddism, or inborn errors of metabolism.

Vitamin deficiency syndromes develop gradually. Symptoms are commonly nonspecific, and the physical examination is rarely helpful in early diagnosis. Most characteristic physical findings are seen late in the course of the syndrome. Other characteristic physical findings, such as glossitis and cheilosis, are seen with deficiencies of many B vitamins. Such abnormalities suggest the presence of a nutritional deficiency but do not indicate which nutrient is deficient.

Despite the relative ease of meeting the recommended daily allowances with a mixed diet, many adults in the United States take vitamin supplements. In fact, syndromes of vitamin excess may be more common than deficiency syndromes, particularly those due to excess of vitamins A, D, and B6. Most claims for significant health benefits of such supplements, particularly those taken in megadoses, remain unsubstantiated.

Some vitamins can be used efficaciously as drugs. Derivatives of vitamin A are used to treat cystic acne. Niacin can be used to treat hyperlipidemia. Vitamin-responsive inborn errors of metabolism also commonly require pharmacologic doses of vitamins.

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