Amino acids are the building blocks of proteins. N-acetyl-cysteine (NAC) is a sulphur-containing free form amino acid, and comes from the amino acid L-cysteine. N-acetyl-cysteine, which is more stable than L-cysteine, is important for production of the free radical fighter, glutathione. NAC, an antioxidant, is used by the liver and lymphocytes to help detoxify the body. NAC also helps boost protective enzymes in the body.