Niacin (vitamin B3) occurs in the body as two metabolically active coenzymes, NAD (nicotinamide adenine dinucleotide) and NADP (NAD phosphate). The niacin coenzymes NAD and NADP have pervasive roles in energy-related and biosynthetic metabolic processes. At least 200 enzymes depend on these niacin cofactors. The NAD-dependent enzymes are involved in mostly catabolic, oxidative reactions that release energy from carbohydrate, fat, and protein, whereas the NADP-dependent enzymes more commonly function in biosynthesis of such compounds as fatty acids and steroid hormones. Independent of its functions as NAD or NADP, niacin is also involved in the regulation of normal blood lipoprotein and cholesterol levels, and the maintenance of normal vascular tone. Dietary niacin is generally well absorbed and taken up by the liver. Any excess niacin is metabolized by the liver and excreted by the kidneys. Because the liver has a limited capacity to metabolize niacin, high doses of regular, crystalline niacin are often not well tolerated, causing vasodilatory side effects known as skin-flushing and itching.