Background The value of aspirin in primary prevention of cancer and cardiovascular disease (CVD) remains unclear. The aim of this study was to identify women who benefit from alternate-day aspirin with regard to all relevant outcomes, including cancer, CVD and major gastrointestinal bleeding.

Methods Long term follow-up data of 27 939 healthy women with baseline plasma samples in the Women's Health Study, a randomised trial of 100 mg alternate-day aspirin versus placebo, were used to develop competing risks models for individualised prediction of absolute risk reduction of the combination of CVD, cancer and major gastrointestinal bleeding by aspirin.

Results Although aspirin was associated with a modestly decreased 15-year risk of colorectal cancer, CVD, and in some women non-colorectal cancer, aspirin treatment resulted in a negative treatment effect in the majority of women if gastrointestinal bleeding was also taken into account. The excess risk of major gastrointestinal bleeding by aspirin increased with age, but the benefits for colorectal cancer and CVD risk were also greater at higher age. Decision curves indicated that selective treatment of women ≥65 years may improve net benefit compared to treating all, none and prediction-based treatment. The observed 15-year number needed to treat to prevent one event among women ≥65 years was 29 (95% CI 12 to 102).

Conclusions Concurrent evaluation of the absolute effects on cancer, CVD and major gastrointestinal bleeding showed that alternate-day use of low-dose aspirin is ineffective or harmful in the majority of women in primary prevention. Selective treatment of women ≥65 years with aspirin may improve net benefit.

Trial registration number NCT00000479.