A 90-year-old man was referred to our hospital complaining of bilateral lower extremity edema and chronic diarrhea. Laboratory data revealed hypokalemia (K 1.7 mEq/L). He developed diarrhea 6 months after starting lansoprazole. We suspected drug-induced collagenous colitis (CC) from lansoprazole therapy. Colonoscopy revealed no obvious abnormal findings, but biopsies were taken from the sigmoid colon for differentiation of CC. Based on biopsy findings of a subepithelial collagen band, he was diagnosed with CC. In addition, his diarrhea improved 3 days after discontinuing lansoprazole and changing to esomeprazole magnesium hydrate. In the differential diagnosis of chronic diarrhea it is important to review the patient's medical history in detail and to perform a colonoscopy and mucosal biopsy while considering CC as a possible diagnosis.
