N-acetylcysteine is an antioxidant that may help to prevent lung damage in people with idiopathic pulmonary fibrosis (IPF), although its effectiveness is far from decided. The treatment is thought to possibly protect the lungs from injury caused by the disease.
It is usually indicated as an adjuvant therapy for people with abnormal, viscid (sticky) or mucous secretions resulting from various lung conditions. Its more common side effects are mild nausea, stomach upset and vomiting.
IPF is often treated with a combination of three medications: prednisone, azathioprine and N-acetylcysteine. However, in a 2011 update to its guidelines for idiopathic pulmonary fibrosis, The American Thoracic Society  analyzed recent studies and found this combination potentially harmful for IPF patients. Based on the findings in a single trial, the guidelines recommended against the combination, although IPF patients already using it should continue and change only after consultation with a physician and based on their preferences.
Likewise, N-acetylcysteine monotherapy was conditionally recommended as an IPF treatment. Evidence from studies did not find significant improvements in lung functions or quality of life, but also did not find notable changes in adverse outcomes.
Overall, The American Thoracic Society did not recommend stopping or discontinuing N-acetylcysteine monotherapy in those IPF patients already receiving it, but little reason was found to initiate this therapy. The Society also noted little difference  between the inhaled or oral forms of N-acetylcysteine.
A Phase 2 study (NCT02707640) also assessed the safety and tolerability of N-acetylcysteine in 123 IPF patients, ages 40 to 80.  All were on pirfenidone treatment and assigned either oral acetylcysteine or placebo for 24 weeks.
The study, which ended in February 2015, primarily assessed adverse events, but also evaluated changes in lung function, among other measures.  Results suggested that adding N-acetylcysteine to pirfenidone was unlikely to benefit these pulmonary fibrosis patients, as it did not alter the tolerability profile of pirfenidone.
The National Heart, Lung, and Blood Institute continues to support clinical research, using a placebo as a comparative drug, into N-acetylcysteine as a potential pulmonary fibrosis treatment.
