Aspirin has long been associated with a reduced risk of colorectal cancer and, possibly, other digestive tract cancers. The optimal duration and dose for cancer prevention, however, aren't certain. Researchers updated a 2012 systematic review and meta-analysis of observational studies of the chemopreventive role of aspirin in digestive tract cancers.

A total of 113 case-control, nested case-control, and cohort studies published through March 2019 were included in the analysis. The studies showed a significant reduction in the risk of colorectal cancer among those taking aspirin regularly compared with those who did not use aspirin. A linear dose-risk relationship was found: an aspirin dose of 75 to 100 mg per day reduced the risk of colorectal cancer by 10%, whereas a dose of 325 mg per day reduced the risk by 35%, and a dose of 500 mg per day reduced it by 50%. Aspirin also reduced the risk of squamous cell esophageal cancer, adenocarcinoma of the esophagus and gastric cardia, stomach cancer, hepatobiliary tract cancer, and pancreatic cancer. Risk estimates were consistent across sex, geographical area, and other covariates. The favorable effect tended to increase with longer duration of use. Overall, there was no association between aspirin use and head and neck cancers.

The authors note that there are inherent biases in observational studies. In this study, the associations were somewhat stronger in case-control compared with cohort studies, and there was some heterogeneity between studies, with differences in populations, baseline cancer risk, aspirin dose, prevalence of aspirin use, and other factors.