Millions take aspirin to prevent colon cancer. A major review says don’t count on it

Bowel cancer, also called colorectal cancer, ranks among the most common cancers worldwide. Prevention usually focuses on healthy lifestyle choices and routine screening tests. In recent years, scientists have investigated whether widely available medications, including nonsteroidal anti-inflammatory drugs (NSAIDs), might help lower the risk.

NSAIDs such as ibuprofen and aspirin are commonly used to treat pain, inflammation, and fever. But whether these drugs can prevent colorectal cancer before it develops remains unclear and widely debated.

To better understand the evidence, researchers at West China Hospital of Sichuan University in China reviewed 10 randomized controlled trials involving 124,837 participants. They examined whether aspirin or other NSAIDs could reduce the risk of colorectal cancer or precancerous growths (adenomas) in people at average risk. No qualifying trials were found for non-aspirin NSAIDs, so the findings apply only to aspirin.

Limited Short-Term Benefit and Uncertain Long-Term Effects

The review found that aspirin likely does not lower the risk of bowel cancer during the first 5 to 15 years of use. Some studies suggested there could be a protective effect after more than 10-15 years of follow-up, but confidence in that evidence is very low.

Those possible long-term benefits come from observational follow-up periods after the original trials ended. During that time, participants may have stopped taking aspirin, started using it on their own, or begun other treatments. These factors make the results more vulnerable to bias.

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.”

Aspirin Bleeding Risks Start Immediately

The researchers also found strong evidence that daily aspirin increases the risk of serious extracranial hemorrhage and likely raises the risk of hemorrhagic stroke.

Higher doses carry greater danger, but even low-dose (“baby”) aspirin increases the likelihood of bleeding. Older adults and people with a history of ulcers or bleeding disorders may face especially high risks.

Because of this, the authors stress that any possible long-term cancer benefit must be carefully weighed against the immediate and well-established bleeding risk.

“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 Approach

Earlier research has shown that aspirin may help certain high-risk groups, including people with inherited conditions such as Lynch syndrome that increase colorectal cancer risk. However, this review focused only on individuals at average risk, and for them, the long-term evidence was highly uncertain.

The authors advise against starting aspirin for cancer prevention without first discussing personal bleeding risks with a healthcare professional.

“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.”

Overall, the researchers conclude that aspirin’s role in cancer prevention is more complicated than once thought. The balance between benefits and harms can shift 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.”