NAC (N-acetylcysteine) has long been used for chronic bronchitis. A study published in 2000, for example, surveyed 9 well-designed trials of NAC for broncopulmonary conditions and found evidence that “a prolonged course of oral NAC prevents acute exacerbations of chronic bronchitis” –meaning improved quality of daily life for patients.
For more details on NAC, see the NYBC entry:
Reference: Grandjean, et al. Clin Ther. 2000 Feb;22(2):209-21.
Efficacy of oral long-term N-acetylcysteine in chronic bronchopulmonary disease: a meta-analysis of published double-blind, placebo-controlled clinical trials. Clin Ther. 2000 Feb;22(2):209-21.