Background Concomitant use of proton-pump inhibitors (PPIs) has been implicated in diminished antiplatelet response to clopidogrel and an increased risk of ischemic events, but primarily among patients undergoing percutaneous coronary intervention. We sought to examine the potential influence of interactions between PPIs and clopidogrel versus prasugrel on platelet reactivity and clinical outcomes after acute coronary syndromes (ACS) in patients managed medically without revascularization. Methods This analysis from the TRILOGY ACS trial focused upon the 7,243 ACS patients aged <75 years who were managed without revascularization, randomized to clopidogrel or prasugrel, and followed for a median of 17 months. Proton-pump inhibitor type and use were assessed at each study visit, and 2,049 of the patients in this cohort underwent serial platelet reactivity assessments. Results Proton-pump inhibitor use (23%) was similar between the clopidogrel and prasugrel groups at baseline and throughout the study. Median on-treatment platelet reactivity values were consistently lower with prasugrel versus clopidogrel irrespective of PPI use. For the primary end point (composite of cardiovascular death, myocardial infarction MI, or stroke), PPI use modified the unadjusted treatment effect of prasugrel versus clopidogrel (interaction P = .02). After adjusting for differences in baseline characteristics, this treatment effect modification was attenuated for the composite end point (interaction P = .06) but was significant for the MI component end point (interaction P = .01). Similarly, among patients on a PPI, the frequency of MI was significantly lower with prasugrel versus clopidogrel (hazard ratio = 0.61; 95% CI 0.42-0.88). These findings were similar by PPI type (omeprazole and pantoprazole). Conclusions Among ACS patients managed without revascularization, use of PPIs did not result in a differential antiplatelet response between prasugrel versus clopidogrel but was associated with a lower incidence of MI with prasugrel. These hypothesis-generating findings suggest that factors besides platelet reactivity may underlie the differential risk of MI observed by treatment assignment with PPI use.