* Dietary Supplement * Antioxidant Support N-Acetyl cysteine is a stable form of the sulfur amino acid L-cysteine, and is a powerful antioxidant. It is also a precursor of glutathione, a major antioxidant. Glutathione is activated by the selenium-containing 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.