A nutrient in the vitamin B complex that the body needs in small amounts to function and stay healthy. Niacin helps some enzymes work properly and helps skin, nerves, and the digestive tract stay healthy. Niacin is found in many plant and animal products. It is water-soluble (can dissolve in water) and must be taken in every day. Not enough niacin can cause a disease called pellagra (a condition marked by skin, nerve, and digestive disorders). A form of niacin is being studied in the prevention of skin and other types of cancer. Niacin may help to lower blood cholesterol. Also called nicotinic acid and vitamin B3.