Background: Aspirin use has been associated with significant reductions in breast cancer-related mortality in some observational studies. However, these studies included women who initiated aspirin use before breast cancer diagnosis. It is unclear whether initiating aspirin use after diagnosis is associated with similar reductions in mortality. This study investigates associations between de novo post-diagnostic aspirin use and all cause, breast cancer-specific mortality. Methods: Women, ages 50 to 80, with a diagnosis of stage I-III breast cancer were identified from Ireland's National Cancer Registry (N = 4,540). Initiation of de novo post-diagnostic aspirin use was identified from linked national prescription refill data (N = 764). Adjusted HRs were estimated for associations between de novo aspirin use and all-cause, breast cancer-specific mortality. Results: The median time from diagnosis to aspirin initiation was 1.8 years. The mean number of days' supply of aspirin received was 631, and 95% of users were taking less than 150 mg/d. We found no association between de novo aspirin use and breast cancer-specific mortality [HR, 0.98; 95% confidence interval (CI), 0.74-1.30]. Similar null associations were found in women taking aspirin at high-intensity (HR, 1.03; 95% CI, 0.72-1.47) and women initiating use in the 1.5 years after diagnosis (HR, 1.04; 95% CI, 0.77-1.40). There was no effect modification by estrogen (Pinteraction = 0.81) or progesterone (Pinteraction = 0.41) receptor status. Conclusion: Initiating aspirin use after a breast cancer diagnosis was not associated with a reduction in breast cancer-specific mortality. Impact: On the basis of our findings, we suggest that a clearer understanding of aspirin's mechanism of action is needed to help inform the design of future studies in breast cancer.
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