BACKGROUND AND AIMS: Aspirin is considered mandatory after myocardial infarction (MI). However, its long-term efficacy has been questioned. This study investigated the effectiveness of long-term aspirin after MI.

METHODS: Patients ≥ 40 years with MI from 2004-2017 who were adherent to aspirin one year after MI were included from Danish nationwide registries. At 2, 4, 6, and 8 years after MI, continued adherence to aspirin was evaluated. Absolute and relative risks of MI, stroke, or death at 2 years from each timepoint were calculated using multivariable logistic regression analysis with average treatment effect modeling standardized for age, sex, and comorbidities. Subgroup analyses were stratified by sex and age > and ≤ 65 years.

RESULTS: Among 40 114 individuals included, the risk of the composite endpoint was significantly higher for nonadherent patients at all timepoints. The absolute risk was highest at 2-4 years after MI for both adherent (8.34%, 95% confidence interval [CI]: 8.05-8.64%) and nonadherent patients (10.72%, 95% CI: 9.78-11.66%). The relative risk associated with nonadherence decreased from 4 years after index-MI and onwards: 1.41 (95% CI: 1.27-1.55) at 4-6 years and 1.21 (95% CI: 1.06-1.36) at 8-10 years (Ptrend = 0.056). Aspirin nonadherence in women and individuals > 65 years was not associated with increased risk. Pinteraction at each of the timepoints: Age-<0.001, <0.001, 0.002, 0.51; Sex - 0.25, 0.02, 0.02, 0.82.

CONCLUSION: Nonadherence to long-term aspirin was associated with increased risk of MI, stroke, or death, but not in women or individuals > 65 years. The risk decreased from 4 years after MI with near statistical significance.