Niacin (nicotinic acid, vitamin B3, Niaspan) - Vitamin AND lipid drug

• water-soluble vitamin that is synthesized by the human body from tryptophan, an essential amino acid that has many dietary sources and is also a precursor to serotonin
• this process is rather inefficient, so dietary sources are important as well
• serves as a precursor to the coenzymes NAD+/NADH and NADP+/NADPH, which serve as electron carriers in the grand process that is cellular respiration (energy production)
• can also be produced by the oxidation of nicotine (thus nicotinic acid), but is most commonly referred to as niacin (nicotinic acid + vitamin) to dissociate it from nicotine, which has obvious negative connotations (see: tobacco addiction)
• a deficiency in this vitamin can result in pellagra, a disease that features "the four D's": diarrhea, dermatitis, dementia, and DEATH
• in addition to being a vitamin (in milligram doses), is employed in gram doses as a broad-spectrum lipid drug that lowers plasma cholesterol levels
• is used to treat various dyslipidemias, many of which promote atherosclerosis and heart disease
• reduces the plasma levels of low density lipoprotein (LDL, "bad" cholesterol) by inhibiting lipolysis (breakdown of fat) in adipose tissue, which decreases plasma levels of free fatty acids, which in turn reduces the secretion of very low density lipoprotein (VLDL), the precursor to LDL, by the liver
• also increases the plasma levels of high density lipoprotein (HDL, "good" cholesterol)
• appears to do these things by acting via a specific receptor on fat cells and stimulating the expression of a particular cholesterol transporter
• has been used by individuals trying to avoid a positive drug test result for fat soluble drugs like marijuana, but there is no scientific basis to this use AND you risk overdosing yourself, which can kill you or cause serious harm (liver damage, arrhythmias, metabolic acidosis, etc.)
• has been shown to cause birth defects in lab animals at pharmacological doses
  

Carlson LA. Nicotinic acid: the broad-spectrum lipid drug. A 50th anniversary review. J Intern Med. 2005 Aug;258(2):94-114.
http://en.wikipedia.org/wiki/Niacin