Colorectal cancer affects millions
Colorectal cancer (cancer of the colon or rectum) is the third most common cancer worldwide. It claims hundreds of thousands of lives every year. Finding a simple, cheap way to prevent it — like a daily aspirin — would be enormously valuable. That's why this research question has been studied for so long.
Aspirin works by reducing inflammation in the body. Since inflammation plays a role in how colon cancer develops, scientists thought aspirin might interrupt that process. Early studies hinted at promise. But the evidence never fully lined up.
What the old thinking promised
For years, some guidelines suggested aspirin might be worthwhile for certain people at higher risk of colorectal cancer. The reasoning made sense: aspirin reduces inflammation, and colon polyps (the small growths that can turn into cancer) are driven by inflammation.
But here's the twist — this new review looked at 10 large, rigorous clinical trials with over 124,000 participants, and the picture that emerged was much less clear.
Aspirin appears to have little to no effect on colon cancer risk for the first 15 years. That's not what many people expected.
Aspirin blocks an enzyme called COX (cyclooxygenase), which the body uses to make inflammation signals. Think of COX as a factory that produces the chemical messages that tell your body to inflame tissue. Aspirin shuts that factory down — at least temporarily.
The theory was that by lowering inflammation over many years, aspirin might prevent the cellular changes that eventually lead to cancer. And maybe it does — but the evidence now suggests it takes a very long time, possibly more than 15 years, before any benefit appears.
The review pooled data from trials where people took low-dose aspirin (75 to 100 mg daily — the standard "baby aspirin" dose) compared to placebo or no treatment. The results were tracked over different time windows: 5 to 10 years, 10 to 15 years, and beyond 15 years.
For the first 15 years, aspirin showed essentially no benefit in reducing colorectal cancer cases. In fact, one large study showed a possible increase in colon cancer deaths within the first 5 to 10 years — though that finding came from just one trial and is uncertain. Only after 15 or more years did the data hint at a possible benefit.
But that potential long-term benefit comes from observational data — the weakest type of evidence — not from the original controlled trials.
The bleeding risk is real and proven
Here's where the evidence becomes very clear — and not in aspirin's favor. Aspirin significantly raises the risk of serious bleeding outside the brain (like gastrointestinal bleeds) and probably raises the risk of hemorrhagic stroke (a stroke caused by bleeding in the brain).
This is high-certainty evidence. It is not disputed. For every potential benefit aspirin might offer against colon cancer — especially over such a long, uncertain timeline — it creates a definite, measurable harm in the near term.
That's not a trade-off most people would knowingly accept.
This research does not mean you should stop aspirin if your doctor has prescribed it for another reason, like heart disease prevention. Those decisions involve a different calculation entirely.
But if you have been thinking about starting a daily aspirin specifically to prevent colon cancer, this review strongly suggests you should have that conversation with your doctor before doing so. The benefit is uncertain and distant. The risks are real and present. Your doctor can help weigh your personal bleeding risk and cancer risk together.
The evidence is complicated by the fact that most long-term data came from the observational follow-up phases of trials — meaning participants were no longer being controlled or blinded. This introduces the possibility that other factors explain the results. The studies were also mostly conducted in Europe and North America, which limits how well the findings apply globally.
The Cochrane reviewers are clear: more well-designed, longer-term trials are needed before any definitive recommendation can be made. In the meantime, researchers continue to investigate whether specific groups of people — based on genetics, cancer risk score, or cardiovascular history — might benefit more from aspirin than others. Personalized medicine may eventually identify who, if anyone, should take aspirin specifically for colorectal cancer prevention. Until then, individualized conversations between patients and doctors remain the most important tool available.