N-Acetyl-L-Cysteine, or NAC, is an amino acid that acts as an antioxidant and is a precursor to glutathione (GSH). While glutathione can be described as the most important endogenously produced antioxidant in the human body, it is also a (tri)peptide, and the body has a tendency to cleave peptides into their individual units. In the case of glutathione, those individual amino acid units are glutamic acid, glycine and cysteine. Of these, cysteine is the most critical for GSH synthesis, and N-acetyl-l-cysteine is an excellent form of cysteine. Therefore, while there is still a school of thought that believes supplemental glutathione can be effective, the fact remains that the most important component for the synthesis of glutathione is NAC. NAC is also more bioavailable than glutathione and is approximately 6 times as cost-effective. However, it is in the role of glutathione precursor that NAC exerts most of its beneficial effects.

From Research Page: http://www.aor.ca/html/products.php?id=140