Not all aspirin products have equivalent antiplatelet efficacy—Aspirin formulated with magnesium stearate is less effective in preventing ischemic stroke

Purpose: Magnesium stearate (MgSt) is a widely used excipient in pharmaceutical formulations but should be avoided in aspirin preparations as it hydrolyzes aspirin. We hypothesized that preparations of aspirin-containing MgSt (MgSt-ASA) are less effective in preventing thrombosis in clinical settings. The risk of composite cardiovascular events in patients treated with MgSt-ASA preparations for preventing secondary stroke was evaluated. Methods: This retrospective cohort study used Taiwan's claims data from 1997 to 2013. Patients who were discharged after ischemic stroke (IS) and administered with only MgSt-ASA or non-MgSt-ASA preparations were enrolled. Composite events including all-cause mortality, IS hospitalization, and myocardial infarction-related hospitalization in the follow-up period under therapy with MgSt-ASA or non-MgSt-ASA preparations were considered primary outcomes. Hazard ratios (HRs) were adjusted with the baseline comorbidities and medications using the Cox model. Results: A total of 19 500 patients with IS (60% males, average age 67 years) were identified, which included 2064 patients receiving MgSt-ASA treatment initially and 17 436 patients receiving non-MgSt-ASA preparation initially. The crude incidence of composite events was 11.65 per 100 person-years, whereas it was 11.45 and 13.90 per 100 person-years for patients receiving non-MgSt-ASA and MgSt-ASA treatments, respectively. The risk of composite events was higher in patients receiving MgSt-ASA preparations than in those receiving non-MgSt-ASA formulations, with the adjusted HR being 1.23 at 95% confidence interval of 1.02 to 1.47. Conclusions: MgSt-ASA preparation use was associated with a higher risk of composite events than non-MgSt-ASA preparations. Review of aspirin formulations under regulatory intervention is warranted.