Incident cancer risk after the start of aspirin use: results from a Dutch population-based cohort study of low dose aspirin users.

Observational and intervention studies suggest that low dose aspirin use may prevent cancer. The objective of this study was to investigate the protective effect of long term low dose aspirin use (≤100 mg daily) on cancer in general and site-specific cancer among low dose aspirin users in the Dutch general population. We conducted a population-based cohort study with detailed information on aspirin exposure and cancer incidence. Only incident (new) low dose aspirin users, who were included in the linkage between PHARMO and the Eindhoven Cancer Registry (1998-2010) and free of cancer before the start of follow up were included. A Cox proportional hazard model with cumulative aspirin use as a time-varying determinant was used to obtain hazard ratios (HR). Duration of aspirin use amongst 109,276 incident low dose aspirin users was not associated with a decreased risk of any of the site-specific cancers or cancer in general (adjusted HR per year of aspirin use for all cancers: 1.02, 95% confidence interval [CI] 1.00-1.04, HR of >6 years aspirin use compared to <2 years: 1.17, 95% CI 1.02-1.34). After adjusting for current and past aspirin use, 2-6 years of low dose aspirin use was associated with a reduced colorectal cancer risk compared to <2 years of aspirin use (adjusted HR 0.75, 95% CI 0.59-0.96). However, a clear dose-response relationship was not observed (adjusted HR >6 years aspirin use 0.95, 95% CI 0.60-1.49). Our results do not support the primary prevention of cancer among long term aspirin users.