Natural Factors N-Acetyl-L-Cysteine (NAC) is a free form amino acid required by the body to make glutathione. Glutathione is a key antioxidant that is involved in many physiological processes, such as fighting oxidative stress, repairing tissues, and supporting overall health. It also recycles other antioxidants and helps reduce the severity and frequency of flu-like symptoms in adults over 65.