Abstract: Objective To analyze such adverse reactions as severe interstitial lung disease (ILD) caused by S-1 (tegafur gimeracil oteracil potassium) capsules in a patient with gastric cancer, and to provide a reference for clinicians. Methods The clinical data of a patient who developed ILD during the treatment of her gastric cancer with oral S-1 was analyzed. The related literature was reviewed and summarized. Results The patient developed fever accompanied by nausea and vomiting three days into the re-treatment with S-1. Chest CT showed that ILD symptoms worsened compared with the previous week. After multidisciplinary consultation and correlation analysis, the patient was diagnosed with S-1-induced ILD. S-1 was discontinued before methylprednisolone and ventilator-assisted ventilation were used. The patient’s clinical symptoms and chest CT findings gradually improved. Conclusion Although S-1-induced ILD is rare, it may lead to severe respiratory failure and is even life-threatening. Clinicians should be alert to the risk of adverse drug reactions and conduct a safety assessment before prescribing S-1. Related examinations should be completed soon after patients develop such symptoms as fever or dyspnea. If S-1-induced ILD is considered likely, clinicians should adopt the right treatment regimen according to the ILD grade to improve the patient’s prognosis.

Key words: Tegafur Gimeracil Oteracil Potassium, Methylprednisolone, Interstitial Lung Disease, Gastric Cancer, Adverse Drug Reactions