N-Acetyl-L-Cysteine (NAC) is a derivative of the dietary amino acid L-cysteine. NAC has a high affinity for lung tissue, which it supports through mucolytic and antioxidant action. Due to its sulfur content, NAC is able to disrupt disulfide bonds within mucus, thinning and easing its expulsion. NAC is a free radical scavenger and thus supports epithelial cell health and healthy cilia activity in the respiratory tract. NAC is also a precursor to the antioxidant glutathione, and supplemental NAC increases tissue levels of glutathione. As fundamental components of the immune system, lymphocytes rely on glutathione to function properly. Tissues enhanced with glutathione support overall antioxidant protection and help to maintain a healthy immune response. A multicenter, randomized, double-blind trial with 262 participants indicated that NAC supplementation for six months supported upper respiratory tract and immune system health. A recent meta-analysis of eight double-blind, placebo-controlled trials provided additional support for NAC's ability to support respiratory tract health.