Aspirin & Bowel Cancer: Risks, Benefits & Latest Evidence (2025)

A widely used painkiller, aspirin, shows limited benefit in preventing colorectal cancer and carries potentially serious risks, according to a comprehensive review of research published through March 2025. The findings, which analyzed data from nearly 125,000 individuals, suggest that daily aspirin use does not significantly reduce the incidence of new colorectal cancer cases over a 5- to 15-year period.

The review, which encompassed ten randomized controlled trials primarily conducted in Europe and North America, with additional studies in Australia and Japan, found that aspirin may offer a slight reduction in new cases after 15 years of continuous use, but researchers expressed limited confidence in this result. Seven of the studies examined low-dose aspirin, typically 75 to 100 mg daily, while three investigated higher dosages.

While aspirin’s potential to prevent colorectal cancer has been a subject of ongoing investigation, the research indicates a more complex picture than previously understood. The analysis revealed that aspirin may slightly increase the risk of death from bowel cancer in the initial 5- to 10-year period, potentially by accelerating the growth of already advanced cancers. After 15 years or more, aspirin might reduce mortality, but the evidence remains inconclusive.

Crucially, the review highlighted a clear and concerning risk associated with aspirin use: an increased likelihood of serious bleeding events, specifically extracranial hemorrhage – bleeding outside the skull – and hemorrhagic stroke, or bleeding within or around the brain. Researchers stated they were confident in the finding of increased serious extracranial hemorrhage risk.

The study did not find sufficient data to evaluate the effects of other nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, on colorectal cancer risk. The research focused specifically on aspirin as the most widely studied NSAID in this context.

Researchers noted several factors limiting the certainty of their findings. The number of studies available was insufficient to draw definitive conclusions and in some trials, participants were aware of their assigned treatment, potentially influencing the results. Not all studies provided comprehensive data on all aspects of the research question.

The investigation into aspirin’s chemopreventive effects – its ability to prevent cancer development – stems from the understanding that chronic inflammation plays a role in tumor formation. Aspirin and other NSAIDs are known to reduce inflammation, prompting research into their potential cancer-fighting properties. Colorectal cancer often develops from lesions known as colorectal adenomas, and researchers have long sought ways to prevent both the lesions and the cancer itself.

Despite the mixed results, the researchers emphasize the need for further investigation to fully understand the long-term effects of aspirin and to explore whether other anti-inflammatory drugs might offer a more favorable balance of benefits and risks in preventing bowel cancer.