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When it's used as a treatment to increase your HDL cholesterol or correct a vitamin deficiency, niacin is sold in higher doses that are prescribed by your doctor. Prescription-strength niacin includes such drugs as Niacor and Niaspan. Niacin is also available as an over-the-counter supplement.
In terms of meeting basic nutritional needs, taking a multivitamin that includes niacin, or vitamin B-3, is safe in combination with other nutrients. However, if you take niacin supplements to help control your cholesterol, certain combinations should be avoided. Vitamin A as beta-carotene, and vitamins C and E may reduce niacin's ability to improve good cholesterol levels, according to MedlinePlus.