Abstract
Lansoprazole is a proton pump inhibitor widely prescribed for gastroesophageal reflux and benign peptic ulcer disease. According to the manufacturer's package insert (TAP Pharmaceuticals, Lake Forest, IL, USA), the most common side-effects are diarrhea, headache and abdominal pain, which occur in approximately 3% of patients and are reversible with drug discontinuation. An unusual case of microscopic colitis is reported in a previously asymptomatic patient who developed new-onset diarrhea after initiation of lansoprazole. The case is reviewed and possible mechanisms of diarrhea secondary to proton pump inhibitors are discussed.