Daily aspirin not reliable for preventing bowel cancer

Daily aspirin use does not offer a quick or reliable way to prevent bowel cancer in the general population and carries immediate risks of serious bleeding, a new Cochrane review finds.

Bowel cancer, also known as colorectal cancer, is one of the most common types of cancer worldwide. Prevention typically involves following a healthy lifestyle and periodically undergoing routine screening tests. In recent years, researchers have also explored the role of off-the-shelf medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs), in reducing the incidence of bowel cancer.

NSAIDs, which include ibuprofen and aspirin, are commonly used to reduce inflammation, fever, and pain. However, their role in the primary prevention of bowel cancer remains uncertain and controversial.

Researchers from West China Hospital of Sichuan University in China analyzed 10 randomized controlled trials including 124,837 participants, assessing whether aspirin or other NSAIDs could prevent colorectal cancer or precancerous polyps (adenomas) in people at average risk. The team found no suitable trials for non-aspirin NSAIDs, so their conclusions focus exclusively on aspirin.

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

The review found that aspirin probably does not reduce the risk of bowel cancer in the first 5 to 15 years of use. Possible protective effects after more than 10–15 years of follow-up were observed in some studies, but the certainty of this evidence is very low.

These potential long-term benefits come from observational follow-up phases of trials, in which participants may have stopped aspirin, started it independently, or begun other treatments, making the findings 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."

Immediate and well-established risks

The findings also show clear evidence that daily use of aspirin increases the risk of serious extracranial haemorrhage and probably increases the risk of haemorrhagic stroke.

Although higher doses carry the greatest risk, low-dose ("baby") aspirin also raises bleeding risk. Older adults and those with a history of ulcers or bleeding disorders may be particularly vulnerable.

The authors therefore caution that any potential long-term benefit must be weighed against the immediate and well-established risk of bleeding.

"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

Previous evidence has shown potential benefits for people at high genetic risk of colorectal cancer, such as those with Lynch syndrome. However, this review focuses strictly on people at average risk, and the long-term evidence for them proved highly uncertain.

The authors urge that patients should not start taking aspirin for cancer prevention without a careful conversation with their healthcare professional about their personal risk of bleeding.

"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."