Daily Aspirin May Do More Harm Than Good for Cancer Prevention

A major new review challenges the idea that a daily aspirin can reliably prevent bowel cancer in the general population.

For years, aspirin has been floated as a simple, low-cost way to lower colorectal cancer risk. A new Cochrane review argues the reality is much less reassuring for people at average risk. In the trials the reviewers could analyze, taking aspirin every day did not deliver a dependable near-term payoff, while the chance of dangerous bleeding rose right away.

Colorectal cancer often develops slowly, frequently starting as small growths called precancerous polyps (adenomas). That long runway is one reason researchers have looked at everyday medications that affect inflammation, a biological process tied to how some tumors form and progress.

Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used for pain and fever, and aspirin in particular also alters blood clotting by reducing platelet activity. That same clotting effect is a major reason aspirin can cause serious bleeding.

To test whether aspirin can prevent cancer before it starts, researchers from West China Hospital of Sichuan University in China examined 10 randomized controlled trials with 124,837 participants. The studies focused on people at average risk and tracked whether taking aspirin or other NSAIDs reduced colorectal cancer or precancerous polyps (adenomas). No qualifying trials were found for non-aspirin NSAIDs, so the review’s conclusions apply only to aspirin.

Little to no short-term benefit and uncertain long-term effects

Across the first 5 to 15 years of use, the evidence indicates aspirin probably does not reduce the risk of bowel cancer. Some studies reported possible protection after more than 10–15 years of follow-up, but the reviewers rated the certainty of that evidence as very low.

A key complication is where those long-term hints come from. In several cases, the “later” results were drawn from observational follow-up after the randomized portion of a trial ended. During those years, participants could have stopped taking aspirin, started it on their own, or changed other treatments, which makes it harder to isolate aspirin’s true effect and increases the risk of biased conclusions.

Lead author Dr. Zhaolun Cai explains: “While the idea of aspirin preventing bowel cancer in the long run is intriguing, our analysis shows that this benefit is not guaranteed and comes with immediate risks.”

Immediate and well-established risks

While the cancer prevention benefit remained uncertain, the harms were clearer. The review found strong evidence that daily aspirin use increases the risk of serious extracranial hemorrhage, meaning major bleeding outside the brain, and it probably increases the risk of hemorrhagic stroke.

Dose mattered, with higher amounts carrying the greatest danger, but the review did not give low-dose aspirin a free pass. Even low-dose (“baby”) aspirin increased bleeding risk. The risk was especially concerning for older adults and for people with a history of ulcers or bleeding disorders, groups already more prone to gastrointestinal bleeding and other complications.

The authors stress that any possible long-term benefit has to be weighed against bleeding that can begin immediately.

“My biggest worry is that people might assume that taking an aspirin today will protect them from cancer tomorrow,” says Dr. Bo Zhang, senior author. “In reality, any potential preventive effect takes over a decade to appear, if it appears at all, while the bleeding risk begins immediately.”

Not a ‘one-size-fits-all’ solution

Aspirin may still have a different role for specific high-risk groups. Previous evidence has pointed to possible benefit in people with inherited risk, such as those with Lynch syndrome. This review, however, kept its focus on the much larger group of people at average risk, and for them the long-term signal was too uncertain to justify broad use.

Instead of self-prescribing, the authors advise people to talk with a clinician before considering aspirin for cancer prevention, especially since personal bleeding risk can vary widely based on age, medical history, and other medications.

“This review reinforces that we must move away from a one-size-fits-all approach,” says Dr. Dan Cao, senior author. “Widespread aspirin use in the general population simply isn’t 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.”

The research team concludes that the story of aspirin for cancer prevention is far more complex than previously believed and that the balance of benefits and harms changes over time.

“As scientists, we must follow the evidence where it leads,” Dr. Zhang adds. “Our rigorous analysis of the highest-quality trials reveals that the ‘aspirin for cancer prevention’ story is more complex than a simple ‘yes or no.’ The current evidence does not support a blanket recommendation for aspirin use purely to prevent bowel cancer.”

Reference: “Aspirin and other nonsteroidal anti‐inflammatory drugs (NSAIDs) for preventing colorectal cancer and colorectal adenoma in the general population” by Zhaolun Cai, Yang Meng, Wenming Yang, Yihui Han, Dan Cao and Bo Zhang, 26 February 2026, Cochrane Database of Systematic Reviews.
DOI: 10.1002/14651858.CD015266.pub2

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