N-acetyl cysteine is a more stable form of the sulfur amino acid L-cysteine, and is a powerful cysteine and glutathione precursor. Glutathione is a major antioxidant as the cofactor for glutathione peroxidase, one of the most important antioxidant enzymes in the body. Additionally, a meta-analysis of human clinical trials has reported that N-acetyl cysteine supplementation supports respiratory health.