N-Acetyl-L-cysteine (NAC) is a derivative of the amino acid cysteine and a precursor to glutathione, the body's most important antioxidant. It acts as a mucolytic agent, helping to break down mucus in the respiratory tract. NAC has antioxidant properties and can help protect cells from oxidative damage. It is also used as a dietary supplement to support liver health and detoxification processes.