View full size image. Vitamin B3, also called niacin, is one of the eight B-complex water-soluble vitamins. Niacin has a wide range of uses in the body, helping functions in the digestive system, skin and nervous system. Niacin, a name coined from ni cotinic ac id vitam in, comes in several forms, including niacinamide (nicotinamide) and inositol hexanicotinate.