BACKGROUND Nonadherence to antiplatelet therapy after percutaneous coronary intervention (PCI) is common, even in clinical trials. OBJECTIVES The purpose of this study was to investigate the impact of nonadherence to study protocol regimens in the MASTER DAPT (Management of High Bleeding Risk Patients Post Bioresorbable Polymer Coated Stent Implantation With an Abbreviated Versus Prolonged DAPT Regimen) trial. METHODS At 1-month after PCI, 4,579 high bleeding risk patients were randomized to single antiplatelet therapy (SAPT) for 11 months (or 5 months in patients on oral anticoagulation [OAC]) or dual antiplatelet therapy (DAPT) for >= 2 months followed by SAPT. Coprimary outcomes included net adverse clinical events (NACE), major adverse cardiac and cerebral events (MACE), and major or clinically relevant nonmajor bleeding (MCB) at 335 days. Inverse probability-ofcensoring weights were used to correct for nonadherence Academic Research Consortium type 2 or 3. RESULTS In total, 464 (20.2%) patients in the abbreviated-treatment and 214 (9.4%) in the standard-treatment groups incurred nonadherence Academic Research Consortium type 2 or 3. At inverse probability-of-censoring weights analyses, NACE (HR: 1.01; 95% CI: 0.88-1.27) or MACE (HR: 1.07; 95% CI: 0.83-1.40) did not differ, and MCB was lower with abbreviated compared with standard treatment (HR: 0.51; 95% CI: 0.60-0.73) consistently across OAC subgroups; among OAC patients, SAPT discontinuation 6 months after PCI was associated with similar MACE and lower MCB (HR: 0.47; 95% CI: 0.22-0.99) compared with SAPT continuation. CONCLUSIONS In the MASTER DAPT adherent population, 1-month compared with >= 3-month DAPT was associated with similar NACE or MACE and lower MCB. Among OAC patients, SAPT discontinuation after 6 months was associated with similar MACE and lower MCB than SAPT continuation (Management of High Bleeding Risk Patients Post Bioresorbable Polymer Coated Stent Implantation With an Abbreviated Versus Prolonged DAPT Regimen [MASTER DAPT]