© 2015 The Authors

and trial3 evidence suggests that frequent use of aspirin reduces the risk of cancer in several organs, particularly of the gastrointestinal tract. Currently, the evidence is strongest for colorectal cancer, with effect sizes from well-conducted prospective studies converging on about 20 % lower incidence of cancer among daily users compared to non-users of aspirin. Frequent aspirin use is also associated with significantly lower risks of adenocarcinomas and squamous cell carcinomas of the oesophagus.1 The mechanisms through which aspirin may inhibit tumour development remain unclear, although there is widespread consensus that aspirin’s inhibition of cyclo-oxygenase (COX) enzymes is likely to mediate some of the observed effects. COX enzymes (notably the inducible form of COX-2) are over-expressed in colorectal adenomatous polyps and in precursor lesions in other organs, and are known to play a role in carcinogenesis, apoptosis and angiogenesis.4,5 Thus, although regular aspirin use is not formally advocated by any agencies for the primary prevention of cancer, it remains of interest to determine the magnitude of potential benefit of daily aspirin consumption in terms of cancer incidence. We therefore estimated the number and fraction of colorectal cancers that may have been prevented from occurring in Australia in 2010 through prior daily aspirin use. Given the suggestive – but weaker – evidence that frequent aspirin use reduces the risks of oesophageal cancer, we also estimated the number and fraction of those cancers that may have been prevented