The patient was a 48-year-old male undergoing treatment for rheumatoid arthritis and rheumatoid lung disease. He was admitted to this hospital for evaluation of an infiltrative shadow in the right upper lobe of the lung. An elevated inflammatory response and extensive new ground-glass opacities in both lungs were observed following an overnight stay out of the hospital. His symptoms subsided with the first dose of steroid pulse therapy; however, when once again interviewed in detail, it became clear that the patient had taken large doses of “Tonin Sekidome-Eki^®,” an antitussive agent, during his overnight stay elsewhere and had developed drug-induced pneumonia. Worsening of the interstitial shadows was no longer observed after the patient was prohibited from taking oral “Tonin Sekidome-Eki^®.” A drug lymphocyte stimulation test (DLST) yielded negative results, and there are no past reports of drug-induced pneumonia resulting from the antitussive agent “Tonin Sekidome-Eki^®.” In our present patient, however, unintended administration of the culprit drug by the patient himself, i.e., a drug provocation test (DPT), induced a relapse (worsening of symptoms), thereby leading to identification of the culprit drug. Differential diagnoses of existing interstitial pneumonia in patients in an immunosuppressed state include exacerbation of rheumatoid lung disease and opportunistic infections. However, in a patient such as our present case, it is considered important to conduct a detailed interview and to consider a differential diagnosis that includes the possibility of drug-induced pneumonia.