Aspirin to Prevent Cardiovascular Disease: The Association of Aspirin Dose and Clopidogrel With Thrombosis and Bleeding

Background: The optimal aspirin dose for the prevention of cardiovascular events remains controversial. Objective: To assess the incidence of and risk factors for adverse clinical outcomes by investigator-determined aspirin dose in a primary prevention trial. Design: Post hoc observational analyses of data from a doubleblind, placebo-controlled, randomized trial. Setting: Outpatient. Patients: 15 595 patients with cardiovascular disease or multiple risk factors. Intervention: Clopidogrel, 75 mg/d, or placebo, with aspirin, 75 to 162 mg/d, as selected by the investigators. Measurements: Incidence of the composite outcome of myocardial infarction, stroke, or cardiovascular death (efficacy end point), and incidence of severe or life-threatening bleeding (safety end point), at a median of 28 months (interquartile range, 23 to 31 months) of follow-up. Results: Daily aspirin doses were categorized as less than 100 mg (75 or 81 mg) (n = 7180), 100 mg (n = 4961), and greater than 100 mg (150 or 162 mg) (n = 3454). The hazard of the primary efficacy end point was the same regardless of dose (adjusted hazard ratio, 0.95 [95% CI, 0.80 to 1.13] for 100 mg vs. less than 100 mg, and 1.0 [CI, 0.85 to 1.18] for greater than 100 mg vs. less than 100 mg). The hazard of the primary safety end point also did not depend on dose (adjusted hazard ratio, 0.85 [CI, 0.57 to 1.26] for 100 mg vs. less than 100 mg and 1.05 [CI, 0.74 to 1.48] for greater than 100 mg vs. less than 100 mg). In patients also receiving clopidogrel, daily aspirin doses greater than 100 mg seemed to be non- statistically significantly associated with reduced efficacy (adjusted hazard ratio, 1.16 [CI, 0.93 to 1.44]) and increased harm (adjusted hazard ratio, 1.30 [CI, 0.83 to 2.04]). Limitation: The analysis was post hoc, and aspirin use was not randomized or blinded. Conclusion: Daily aspirin doses of 100 mg or greater were associated with no clear benefit in patients taking aspirin only and possibly with harm in patients taking clopidogrel. Daily doses of 75 to 81 mg may optimize efficacy and safety for patients requiring aspirin for long-term prevention, especially for those receiving dual antiplatelet therapy.