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BACKGROUND Gastrointestinal bleeding is the most frequent major complication of antiplatelet therapy. In patients at low bleeding risk, however, clinically overt gastrointestinal bleeding is relatively uncommon. OBJECTIVE We sought to assess the effects of different antiplatelet regimens on gastrointestinal mucosal injury using a novel magnetically-controlled capsule endoscopy system in patients at low bleeding risk. METHODS Patients (n=505) undergoing percutaneous coronary intervention in whom capsule endoscopy demonstrated no ulcerations or bleeding (although erosions were permitted) after 6 months of dual antiplatelet therapy (DAPT) were randomly assigned to aspirin plus placebo (n=168), clopidogrel plus placebo (n=169), or aspirin plus clopidogrel (n=168) for an additional 6 months. The primary endpoint was the incidence of gastrointestinal mucosal injury (erosions, ulceration, or bleeding) at 6-month or 12-month capsule endoscopy. RESULTS Gastrointestinal mucosal injury through 12 months was less with single antiplatelet therapy (SAPT) compared with DAPT (94.3% vs. 99.2%, P=0.02). Aspirin and clopidogrel monotherapy had similar effects. Among 68 patients without any gastrointestinal injury at randomization (including no erosions), SAPT compared with DAPT caused less gastrointestinal injury (68.1% vs. 95.2%, P=0.006), including fewer new ulcers (8.5% vs. 38.1%, P=0.009). Clinical gastrointestinal bleeding between 6 and 12 months was less with SAPT compared with DAPT (0.6% vs. 5.4%, P=0.001). CONCLUSIONS Despite being at low risk of bleeding, nearly all patients receiving antiplatelet therapy developed gastrointestinal injury, although overt bleeding was infrequent. DAPT for 6 months followed by SAPT with aspirin or clopidogrel between 6 and 12 months resulted in less gastrointestinal mucosal injury and clinical bleeding compared with DAPT through 12 months.