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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.
Common side effects of niacin include flushing, itching, and gastrointestinal disturbances, such as nausea and vomiting. Hepatotoxicity (liver cell damage), including elevated liver enzymes and jaundice, has been observed at intakes as low as 750 mg of niacin per day for less than three months (84, 85).