Niacin is an organic compound and vitamin.

Niacin is a water-soluble vitamin, which is also known as nicotinic acid or vitamin B3. Nicotinamide is the derivative of niacin and used by the body to form the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). The chemical structure of the various forms of niacin are shown in the diagram. None of the forms are related to the nicotine found in tobacco, although their names are similar. It is believed to "promote metabolism" of the drug and cause it to be "flushed out." Scientific studies have shown it does not affect drug screenings, but can pose a risk of overdose, causing arrhythmias, metabolic acidosis, hyperglycemia, and other serious problems. Slow- or "sustained"-release forms of niacin have been developed to lessen these side-effects.

Niacin is converted to niacinamide in vivo, and though the two are identical in their vitamin functions, niacinamide does not have the same pharmacologic and toxic effects of niacin, which occur incidental to niacin's conversion. Thus niacinamide does not reduce cholesterol or cause flushing, although nicotinamide may be toxic to the liver at doses exceeding 3 g/day for adults. Niacin is a precursor to NADH, NAD, NAD+, and NADP, which play essential metabolic roles in living cells. DNA repair, and the production of steroid hormones in the adrenal gland. Niacin is sometimes consumed in large quantities by people who wish to fool drug screening tests, particularly for lipid soluble drugs such as marijuana.

Niacin, when taken in large doses, blocks the breakdown of fats in adipose tissue, thus altering blood lipid levels. Niacin is used in the treatment of hyperlipidemia because it reduces very-low-density lipoprotein (VLDL), a precursor of low-density lipoprotein (LDL) or "bad" cholesterol. Because niacin blocks breakdown of fats, it causes a decrease in free fatty acids in the blood and, as a consequence, decreased secretion of VLDL and cholesterol by the liver. By lowering VLDL levels, niacin also increases the level of high-density lipoprotein (HDL) or "good" cholesterol in blood, and therefore it is sometimes prescribed for patients with low HDL, who are also at high risk of a heart attack.

Niacin has long been used to increase high-density lipoprotein (HDL), the "good" cholesterol. But compared with other cholesterol drugs, niacin hasn't gotten much respect. It's just a simple B vitamin, nothing fancy. Still, it's hard to deny niacin's often-significant impact on your HDL cholesterol levels. A lot of the attention regarding cholesterol has been focused on lowering your low-density lipoprotein (LDL), or "bad" cholesterol. That's still an important goal.