Niacin is the compound nicotinic acid, and is a term to describe a number of compounds related to nicotinic acid, including nicotinamide (niacinamide) and inositol nicotinate (inositol hexaniacinate). The different forms of niacin are not interchangeable and have different medical purposes. Niacin (nicotinic acid) is commonly used to treat high cholesterol. Nicotinamide is taken to treat arthritis and pellagra. Nicotinamide is also used as a vitamin supplement and does not cause the flushing that is commonly associated with niacin.