Cysteine is an amino acid that serves as a precursor to glutathione, a major intracellular antioxidant. Its use to support mucus membranes, particularly in the respiratory tract, is primarily supported by scientific evidence related to its role as a mucolytic agent. N-acetylcysteine (NAC), a derivative of cysteine, is widely used in clinical settings to reduce the viscosity of mucus, making it easier to expectorate in conditions such as chronic bronchitis, chronic obstructive pulmonary disease (COPD), cystic fibrosis, and other respiratory conditions characterized by thick mucus. This effect is achieved through the breaking of disulfide bonds in mucoproteins, thereby reducing mucus thickness and improving mucociliary clearance.

Multiple clinical trials and systematic reviews have demonstrated that NAC can improve lung function and decrease exacerbations in patients with chronic respiratory diseases. Its antioxidant properties also contribute to reducing oxidative stress and inflammation in the mucus membranes of the respiratory tract. While cysteine itself is less commonly used directly as a supplement compared to NAC, the scientific rationale and clinical data supporting NAC's benefits are robust and directly relevant to mucus membrane health, particularly in the airways. Thus, the use of cysteine (or its derivative NAC) for supporting mucus membranes is well-grounded in scientific research.