The cardioprotective effects of aspirin are well established. A meta-analysis of individual subject data from primary prevention randomised controlled trials (RCTs) suggested that aspirin can reduce the relative risk of non-fatal myocardial infarction by about 20%.1 Overall, however, the risks of treatment (severe gastrointestinal and extracranial bleeding) were roughly the same as the benefits, so routine use of aspirin as a primary preventive strategy was not recommended. The meta-analysis did not evaluate any potential reduction of mortality from cancer, as has been suggested by observational data.2 Observational data are difficult to interpret, however, because associations may not be causal and may be the result of confounding or bias.3 Rothwell and colleagues have therefore conducted another meta-analysis of individual subject data from RCTs of aspirin versus no aspirin for prevention of vascular disease, but this time they evaluated mortality from cancer as the main outcome.4 They found a 21% (95% confidence interval 8% to 32%) reduction in the odds of death from cancer in people who took aspirin for almost six years, and the effect was strongest for gastrointestinal cancers. The authors …