Niacin is a form of vitamin B3. It is found in foods such as yeast, meat, fish, milk, eggs, green vegetables, and cereal grains. Niacin is also produced in the body from tryptophan, which is found in protein-containing food. When taken as a supplement, niacin is often found in combination with other B vitamins. Do not confuse niacin with niacinamide, inositol niacinamide, nicotinate (inositol hexaniacinate), or tryptophan.