Eight RCTs (n=25,570 participants) were included in the initial analysis. IPD were available for seven RCTs (n=23,535 participants), six of which were placebo-controlled and double-blind. Three trials (n=12,659 participants) provided an analysis of 20-year risk of cancer death.
Pooled data for all trials showed that daily aspirin was associated with a reduction in deaths due to cancer (OR 0.79, 95% CI 0.68 to 0.92; eight trials; 674 cancer deaths; non-significant heterogeneity). After five years' follow-up, pooled IPD revealed a significant benefit from daily aspirin on all cancers (HR 0.66, 95% CI 0.50 to 0.87; seven trials; 657 cancer deaths) and for gastrointestinal cancers (HR 0.46, 95% CI 0.27 to 0.77).
Pooled analysis of trials that assessed 20-year risk of cancer death showed significantly lower risk in patients who took aspirin in terms of all cancers (HR 0.78, 95% CI 0.70 to 0.87), all solid cancers (HR 0.80, 95% CI 0.72 to 0.88) and gastrointestinal cancers (HR 0.65, 95% CI 0.54 to 0.78). Subgroup analysis revealed that benefits significantly increased for all solid cancers and gastrointestinal cancers with extended scheduled treatment duration (7.5 years or longer). Further subgroup analyses revealed that the effect on deaths was observed only after around five years for oesophageal, pancreatic, brain and lung cancers. The time delay was greater for stomach, colorectal and prostate cancers. The greatest benefit in lung and oesophageal cancers was for adenocarcinomas after 20 years' follow-up (HR 0.66, 95% CI 0.56 to 0.77).
The effects of aspirin did not differ significantly by dose (over 75mg) and in relation to gender and smoking status. The absolute reduction in 20-year risk of cancer death was 7.08% (95% CI 2.42 to 11.74) in patients 65 years or older at randomisation.