Pirfenidone is an antifibrotic agent that has been proven to slow down the progression of idiopathic pulmonary fibrosis (IPF). The aim of this study was to evaluate the efficacy of low-dose pirfenidone (that is, less than 1200 mg/day). We retrospectively reviewed the medical records of patients with IPF. The patients were divided into the following three groups, those who were not treated with pirfenidone (control) and those who were treated with pirfenidone at doses < 1200 mg/day (low-dose group) and ≥ 1200 mg/day (high-dose group). The adjusted mean changes in forced vital capacity (FVC) in 1 year were - 200.7, - 88.4, and - 94.7 mL in the control, low-dose, and high-dose groups (p = 0.021). The FVC declined more significantly in the control group than in the low-dose and high-dose groups. No significant difference in FVC change was observed between the low-dose and high-dose groups. Dyspepsia, anorexia, and nausea were significantly more frequent in the low-dose than in the high-dose group, suggesting that dose reduction is attributed to gastrointestinal tract-related adverse events. Dose reduction may help patients to better control gastrointestinal tract-related adverse events; continuing taking the medication at low doses is also expected to be effective in reducing the FVC decline.