Aspirin Does Not Reliably Prevent Bowel Cancer and Raises Bleeding Risk Immediately

Daily aspirin use does not reliably prevent bowel cancer in people at average risk - and it raises the risk of serious bleeding immediately. That is the unambiguous conclusion of a Cochrane systematic review analyzing 10 randomized controlled trials with 124,837 participants, conducted by researchers at West China Hospital of Sichuan University and published in 2026.

The review focused specifically on aspirin and colorectal cancer prevention in average-risk adults - excluding people at elevated genetic risk such as those with Lynch syndrome, where a different body of evidence applies. No suitable trials of non-aspirin NSAIDs were found, so the conclusions address aspirin only.

What the evidence shows across different time horizons

The short-term picture is clear: aspirin probably does not reduce bowel cancer risk in the first 5 to 15 years of use. The long-term picture is murkier, and the murkiness is the problem. Some studies have suggested a possible protective effect after more than 10 to 15 years of use - but these signals come from observational follow-up phases of trials, in which participants may have stopped aspirin, started it independently, or begun other treatments. These phases are vulnerable to the kinds of bias that randomized trials are designed to eliminate. The certainty of this long-term evidence is rated as very low.

Senior author Dr. Bo Zhang summarized the temporal mismatch plainly: "Any potential preventive effect takes over a decade to appear, if it appears at all, while the bleeding risk begins immediately."

Bleeding risk is not theoretical

The evidence on harm is substantially more certain than the evidence on benefit. Daily aspirin use clearly increases the risk of serious extracranial hemorrhage - primarily gastrointestinal bleeding - and probably increases the risk of hemorrhagic stroke. Both low-dose and standard-dose aspirin carry this risk, though the magnitude increases with dose. Older adults and those with histories of ulcers or bleeding disorders face the greatest exposure.

This is not a minor concern. Gastrointestinal bleeding from aspirin use is the reason widespread aspirin recommendations for cardiovascular prevention have been walked back in recent years by major guidelines. The Cochrane review makes the same case for cancer prevention: the established immediate harm profile sets a high evidential bar for any proposed long-term benefit that the current data cannot meet.

What this means for patients at average risk

The review does not address high-risk populations, for whom different evidence applies. For people at average bowel cancer risk - the majority of adults considering aspirin for prevention - the data do not support routine use. Lead author Dr. Zhaolun Cai noted the risk of misinterpretation: "The idea of aspirin preventing bowel cancer in the long run is intriguing, but this benefit is not guaranteed and comes with immediate risks."

Senior author Dr. Dan Cao pointed toward precision prevention as a more promising direction: "Widespread aspirin use in the general population simply is not supported by the evidence. The future lies in precision prevention - using molecular markers and individual risk profiles to identify who might benefit most and who is most at risk." Until that framework exists for bowel cancer prevention, the evidence supports a straightforward recommendation: patients should not start taking aspirin for cancer prevention without a careful discussion of their personal bleeding risk with a healthcare professional.