N-Acetyl-L-Cysteine, or NAC, is an amino acid that acts as an antioxidant and is a precursor in the body to glutathione (GSH), a critical antioxidant. NAC is a more stable form of the amino acid cysteine (which is very unstable and oxidizes readily) because it has an acetyl group (CH3CO) attached. NAC is also more bioavailable and 6 times as cost-effective as the tripeptide glutathione. Glutathione consists of three amino acids: cysteine, glutamic acid and glycine. The body breaks this tripeptide apart before absorption, but the rate limiting (most valuable) amino acid for the purpose of making glutathione is the cysteine. Therefore, simply taking NAC makes much more sense.