N-acetyl cysteine is a more stable form of the sulfur amino acid l-cysteine, and is a powerful antioxidant. It is an excellent precursor of glutathione, another major antioxidant. Glutathione is also the precursor, with selenium, of glutathione peroxidase, one of the most important antioxidant enzymes in the body. N-acetyl cysteine is better than glutathione supplements which may break down during digestion, and lose potency.