Background: Vonoprazan, a potassium-competitive acid blocker, inhibits gastric acid secretion and attenuates the antiplatelet function of clopidogrel more potently than esomeprazole. We investigated whether alternate-day dosing of vonoprazan might avoid this interaction with clopidogrel while providing sufficient gastric acid inhibition. Methods: Following 24 h of pH monitoring (control regimen), 12 healthy volunteers received three regimens (clopidogrel-only regimen: clopidogrel 75 mg daily [q.d.]; vonoprazan alternate-day regimen: vonoprazan 10 mg every other day [q.o.d.] + clopidogrel 75 mg q.d.; vonoprazan daily regimen: vonoprazan 10 mg q.d. + clopidogrel 75 mg q.d.) for 14 days in a randomized open-label crossover manner. Intragastric pH monitoring was performed for 24 h on day 13 in the clopidogrel-only and vonoprazan q.d. regimens and for 48 h on days 13 and 14 in the vonoprazan q.o.d. regimen. Serum gastrin and inhibition of platelet aggregation (IPA) were measured before the commencement of pH monitoring in each regimen. Results: Twelve volunteers completed the study. Equivalent median IPA values in the q.o.d. and q.d. regimens were measured (21.8% and 25%, respectively) and were significantly lower than that with the clopidogrel-only regimen (40.8%). The median pH4 holding time ratio for the vonoprazan q.o.d. regimen (49.7%) was superior to that of the clopidogrel-only regimen (18.4%), but was significantly inferior to that of the vonoprazan q.d. regimen (77.0%; p < 0.01). Conclusion: Alternate-day administration of vonoprazan could not prevent the interaction between vonoprazan and clopidogrel, and acid inhibition was inferior to that with vonoprazan daily administration. Alternate-day administration of vonoprazan thus appears to be of questionable clinical utility.