N-Acetyl-Cysteine, or NAC, is a more stable form of the amino acid, l-cysteine. NAC is considered both an amino acid and an antioxidant. NAC forms the antioxidant, glutathione, which is important to the liver. NAC is an important part of proper body function, as the amino acid is connected to metabolism as well respiration support. A health care professional should be consulted before using.