L-Acetyl n-Cysteine (NAC) is an acetylated form of the amino acid cysteine and is used medically as a mucolytic agent. For emphysema, a chronic obstructive pulmonary disease (COPD) characterized by airway inflammation and mucus production, NAC's primary role is to reduce mucus viscosity and support expectoration. Scientific evidence for NAC in COPD, which includes emphysema, is moderate. Several randomized controlled trials and meta-analyses have shown that oral NAC (generally at doses of 600–1200 mg daily) can reduce the frequency and severity of exacerbations in chronic bronchitis and COPD patients. However, the magnitude of benefit specifically in emphysema is less clear, as trials often group emphysema and chronic bronchitis together under COPD.

The proposed mechanisms of NAC's benefit are twofold: as a mucolytic, it breaks disulfide bonds in mucus glycoproteins, making secretions less viscous; as an antioxidant, it replenishes intracellular glutathione, which may help counteract oxidative stress implicated in lung tissue damage and inflammation in emphysema. The 2015 Cochrane review (Cochrane Library) concluded that NAC may have a modest impact in reducing exacerbations, though the evidence quality is variable and not all studies show benefit.

In summary, there is scientific validation for the use of NAC in COPD, including emphysema, primarily for reducing exacerbations and thinning mucus. It is not a disease-modifying therapy but may support symptomatic management. The evidence is rated moderate (3/5) due to heterogeneity in study results and limited data specific to emphysema.