Association between Low-Dose Aspirin Use and Colorectal Cancer Incidence using Japanese Large-scale Health Insurance Claims Data

Background: While experimental studies suggest low-dose aspirin has chemopreventive effects against colorectal cancer, clinical evidence remains inconsistent, and studies on Japanese populations are limited. This study aimed to evaluate the relation between low-dose aspirin and colorectal cancer using a large-scale Japanese database. Methods: A nested case-control study was conducted using a health insurance claims database (2005–2021). Cases were defined as patients with a diagnosis of colorectal cancer during the study period, with the month of the first diagnosis as the index month, and were matched 1:4 risk-set sampling with population controls based on age and sex. Exposure was defined as the use of low-dose aspirin prior to the index month. We further analyzed recency and duration. Recency was categorized as recent use (< 3 months before the index month) and past use (≥ 3 months before the index month). Duration was classified into two groups: < 2 years and ≥ 2 years. Results: In total, 17,373 cases and 69,492 controls were included. Conditional logistic regression analysis, adjusted for patient background, showed that low-dose aspirin use may reduce the risk of colorectal cancer (adjusted odds ratio: 0.87, 95% confidence interval: 0.79–0.97). For recency and duration, the risk of colorectal cancer was significantly reduced by recent use and ≥ 2 years compared to never use. Conclusions: Low-dose aspirin use may be associated with a reduced risk of colorectal cancer. Impact: These findings contribute to the clinical evidence regarding colorectal cancer prevention in Asian populations.