Importance Idiopathic pulmonary fibrosis (IPF) has a poor prognosis and limited treatment options. Patients with IPF have altered lung microbiota, with bacterial burden within the lungs associated with mortality; previous studies have suggested benefit with co-trimoxazole (trimethoprim-sulfamethoxazole).

Objective To determine the efficacy of co-trimoxazole in patients with moderate and severe IPF.

Design, Setting, and Participants Double-blind, placebo-controlled, parallel randomized trial of 342 patients with IPF, breathlessness (Medical Research Council dyspnea scale score >1), and impaired lung function (forced vital capacity ≤75% predicted) conducted in 39 UK specialist interstitial lung disease centers between April 2015 (first patient visit) and April 2019 (last patient follow-up).

Interventions Study participants were randomized to receive 960 mg of oral co-trimoxazole twice daily (n = 170) or matched placebo (n = 172) for between 12 and 42 months. All patients received 5 mg of folic acid orally once daily.

Main Outcomes and Measures The primary outcome was time to death (all causes), lung transplant, or first nonelective hospital admission. There were 15 secondary outcomes, including the individual components of the primary end point respiratory-related events, lung function (forced vital capacity and gas transfer), and patient-reported outcomes (Medical Research Council dyspnea scale, 5-level EuroQol 5-dimension questionnaire, cough severity, Leicester Cough Questionnaire, and King’s Brief Interstitial Lung Disease questionnaire scores).

Results Among 342 individuals who were randomized (mean age, 71.3 years; 46 [13%] women), 283 (83%) completed the trial. The median (interquartile range) duration of follow-up was 1.02 (0.35-1.73) years. Events per person-year of follow-up among participants randomized to the co-trimoxazole and placebo groups were 0.45 (84/186) and 0.38 (80/209), respectively, with a hazard ratio of 1.2 ([95% CI, 0.9-1.6]; P = .32). There were no statistically significant differences in other event outcomes, lung function, or patient-reported outcomes. Patients in the co-trimoxazole group had 696 adverse events (nausea [n = 89], diarrhea [n = 52], vomiting [n = 28], and rash [n = 31]) and patients in the placebo group had 640 adverse events (nausea [n = 67], diarrhea [n = 84], vomiting [n = 20], and rash [n = 20]).

Conclusions and Relevance Among patients with moderate or severe IPF, treatment with oral co-trimoxazole did not reduce a composite outcome of time to death, transplant, or nonelective hospitalization compared with placebo.

Trial Registration ISRCTN Identifier: ISRCTN17464641