Long-term follow-up of randomised trials of aspirin in prevention of vascular events showed that daily aspirin reduced the incidence of colorectal cancer and several other cancers and reduced metastasis. However, statistical power was inadequate to establish effects on less common cancers and on cancers in women. Observational studies could provide this information if results can be shown to be reliable. We therefore compared effects of aspirin on risk and outcome of cancer in observational studies versus randomised trials.
For this systematic review, we searched for case-control and cohort studies published from 1950 to 2011 that reported associations between aspirin use and risk or outcome of cancer. Associations were pooled across studies by meta-analysis and stratified by duration, dose, and frequency of aspirin use and by stage of cancer. We compared associations from observational studies with the effect of aspirin on 20-year risk of cancer death and on metastasis in the recent reports of randomised trials.