Key messages
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The evidence on whether daily aspirin can prevent bowel cancer is mixed and uncertain, but there is clear evidence of an increased risk of bleeding in the brain and around the skull.
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Aspirin probably does not make any difference to the number of new bowel cancer cases after 5 to 15 years of follow-up. After 15 years, aspirin may reduce the number of new bowel cancer cases, but we are not confident in this result.
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Aspirin may increase deaths from bowel cancer slightly after 5 to 10 years; it may reduce deaths after 15 years or more, but we are not confident in this result.
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Aspirin probably does not affect the overall number of serious side effects, but it increases the risk of serious extracranial hemorrhage (bleeding that occurs outside the skull) and probably increases the risk of hemorrhagic stroke (bleeding in or around the brain).
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More research is needed to understand the long-term effects of aspirin and to see whether other anti-inflammatory drugs can prevent bowel cancer or not.
What is bowel cancer and chemoprevention?
Bowel cancer, also known as colorectal cancer (CRC), is a common and serious disease. It can sometimes be preceded by lesions (i.e. areas of abnormal growth) known as colorectal adenomas (CRAs). For many years, researchers have been interested in using medications to reduce the risk of CRC and CRA. This approach is known as chemoprevention.
NSAIDs are a group of drugs that help reduce inflammation, fever, and pain, and can also help prevent blood clots. Aspirin and ibuprofen are examples of widely used NSAIDs. They have been studied as a way to help prevent cancer because long-term inflammation is believed to play a role in the development of tumors.
What did we want to know?
We wanted to find out whether aspirin and other NSAIDs are helpful or harmful when used to prevent CRC and CRA in the general population.
What did we do?
We searched for studies known as 'randomized controlled trials' that compared aspirin and other NSAIDs with either no treatment or different treatments for preventing CRC in the general population. We combined the results of these studies (where possible) and rated our confidence in the evidence based on factors such as the size of the studies and the quality of their methods.
What did we find?
We found 10 studies that looked at whether aspirin helps prevent CRC in the general population. These studies included a total of 124,837 people. Most of them took place in Europe and North America, with one study having a site in Australia, and two large studies were carried out in Japan. Seven of the studies tested low-dose aspirin (75 to 100 mg daily (often called 'baby aspirin')), and three evaluated higher doses.
We did not find any studies on the effect of other NSAIDs on the risk of developing CRC or CRA.
What is the effect of aspirin on bowel cancer?
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Aspirin probably does not make a difference to the number of new CRC cases after 5 to 15 years. Aspirin may reduce the number of new CRC cases when taken for 15 years or longer, but we are not confident in this result.
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Aspirin may increase deaths slightly after 5 to 10 years (one hypothesis is that aspirin might speed up the growth of advanced cancers that were already present in some people) and may result in little to no difference in CRC mortality after 10 years and up to 15 years. After 15 years or more of follow-up, aspirin may reduce deaths, but we are not confident in this result.
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Aspirin may not make a difference to the number of new cases of CRA between 5 and 10 years of follow-up, but we are not confident in this result.
What are the risks of taking aspirin to prevent bowel cancer?
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Aspirin probably does not make a difference to the overall number of serious side effects.
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Aspirin increases serious extracranial hemorrhage (bleeding that occurs outside the skull).
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Aspirin probably increases hemorrhagic stroke (bleeding in or around the brain).
How reliable is the evidence we found?
We are confident in the finding of increased serious extracranial hemorrhage risk. We have moderate confidence in the results of new CRC cases after 5 to 15 years, serious side effects, and hemorrhagic stroke. However, we have little to very little confidence in our other findings.
Three main factors reduced our confidence in the evidence.
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There were not enough studies to be sure about the results.
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In some studies, people may have known which treatment they were receiving, which could affect the results.
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Not all the studies provided information about everything that we were interested in.
How up to date is this evidence?
This evidence is based on searches of medical databases that we ran up to March 2025.
Baca abstrak penuh
Matlamat
To assess the benefits and harms of nonsteroidal anti-inflammatory drugs (NSAIDs), including aspirin, for preventing colorectal cancer (CRC) and colorectal adenoma (CRA) in the general population.
Kaedah Pencarian
We searched CENTRAL, MEDLINE, Embase, and two clinical trial registers (ClinicalTrials.gov and WHO ICTRP) on 3 March 2025.
Kesimpulan Pengarang
It is not possible to draw definitive conclusions or outline specific implications for the routine use of aspirin for CRC primary prevention based on the current evidence. Our findings reveal complex, time-dependent preventive effects and concerns about potential harms for clinicians and patients to consider.
Evidence of very low to moderate certainty shows little to no benefit for CRC or CRA incidence in the first 15 years, and low-certainty evidence suggests a potential increase in CRC mortality in the first 5 to 10 years. Very low-certainty evidence suggests potential benefits for CRC incidence and mortality after long-term follow-up (≥ 15 years), but these potential long-term benefits are derived from findings in the observational follow-up phases of RCTs, where standard intention-to-treat analyses are not robust to post-randomization confounding from factors such as treatment contamination.
The uncertain and delayed potential for benefit must be weighed against a definite harm. While aspirin probably has little to no effect on overall serious adverse events (moderate-certainty evidence), it increases the risk of serious extracranial hemorrhage (high-certainty evidence) and probably increases the risk of serious extracranial hemorrhage (moderate-certainty evidence).
In light of the mixed evidence, clinical practice should continue to center on an individualized assessment and a shared decision-making process, carefully balancing a patient's established cardiovascular risk profile against their risk of bleeding.
Funding
This Cochrane review was funded (in part) by the China Postdoctoral Science Foundation (2024M752248) and the Postdoctoral Fellowship Program (Grade A) of China Postdoctoral Science Foundation (BX20230244).
Registration
Protocol available via doi.org/10.1002/14651858.CD015266
