A Study to Assess the Safety and Tolerability of N-Acetylcysteine When Administered With Pirfenidone to Participants With Idiopathic Pulmonary Fibrosis (IPF)

Inclusion Criteria: * Clinical symptoms consistent with IPF of \>=3 months' duration (relative to Day 1) * Must have been on a dose of pirfenidone not less than 1602 mg/day for at least 8 weeks prior to randomization at Day 1 * Able to understand the importance of adherence to study treatment and the study protocol and willing to follow all study requirements, including the concomitant medication restrictions, throughout the study * Women of childbearing capacity were required to have a negative serum pregnancy test before treatment and must have agreed to maintain highly effective contraception by practicing abstinence or by using at least two methods of birth control from the date of consent through the end of the study * Diagnosis of usual interstitial pneumonia (UIP) or IPF by high-resolution computed tomography (HRCT) and surgical lung biopsy. Previous HRCT scans, typically and if available, one at the point of time of diagnosis and one more recent, made during the last year before study inclusion, will be used and assessed by a central Reading Committee Exclusion Criteria: * Significant clinical worsening of IPF between screening and Day 1 of study, in the opinion of the investigator * Unlikely to comply with the requirements of this study, in the opinion of the investigator * Patient-reported cigarette smoking within 3 months of screening or unwilling to avoid use of tobacco products throughout the study * History of clinically significant environmental exposure known to cause pulmonary fibrosis (PF), including but not limited to drugs (such as amiodarone), asbestos, beryllium, radiation, and domestic birds * Known cause of interstitial lung disease, including but not limited to radiation, drug toxicity, sarcoidosis, hypersensitivity pneumonitis, and cryptogenic organizing pneumonia * Clinical diagnosis of any connective tissue disease, including but not limited to scleroderma, polymyositis/dermatomyositis, systemic lupus erythematosus, and rheumatoid arthritis * Clinical evidence of active infection, including but not limited to bronchitis, pneumonia, sinusitis, urinary tract infection, or cellulitis (as a diffuse inflammation of connective tissue and or skin) * Any history of malignancy likely to result in significant disability or likely to require significant medical or surgical intervention within the next 6 months (relative to Day 1). This does not include minor surgical procedures for localized cancer (e.g., basal cell carcinoma, squamous skin carcinoma) * History of severe hepatic impairment or end-stage liver disease * History of end-stage renal disease requiring dialysis * History of unstable or deteriorating cardiac or pulmonary disease (other than IPF) within the previous 6 months (relative to Day 1) * Any condition that, in the opinion of the investigator, may have been significantly exacerbated by the known side effects associated with the administration of N-acetylcysteine taken as a single medication * Suspected intolerance, allergy, or hypersensitivity to pirfenidone or any of its components * Known intolerance, allergy, or hypersensitivity to N-acetylcysteine or any of its components