“Aspirin cuts bowel cancer risk by 22%: a tablet a day helps stop killer tumours forming,” read the headline in the Daily Mail. It referred to a study which looked at aspirin use and the risk of developing colorectal (bowel) cancer. The study found that the lowest dose of aspirin (75mg daily) had a protective effect after five years of use in the general population.
This large study supports the results of previous research, which suggest that aspirin can reduce the risk of colorectal cancer. However, it was the first study to look at what dose might be effective and for how long it might need to be taken. The results are important, but this type of study cannot prove by itself that low-dose aspirin reduces the risk of bowel cancer, and there are a number of limitations that may make the results unreliable. Further high-quality research is needed to confirm the results of this study.
Aspirin can have side effects, including stomach ulcers and internal bleeding. It should not be taken regularly without first talking to your doctor. Due to the risk of a serious complication called Reye’s syndrome, aspirin must not be given to anyone under 16 years old without specialist advice.
Where did the story come from?
The study was carried out by researchers from the University of Edinburgh, Napier University and the Western General Hospital in Edinburgh and was funded by Cancer Research UK, the Scottish government Chief Scientist Office, the Medical Research Council and the medical charity Core. It was published in the peer-reviewed medical journal Gut.
The research was reported widely in the media, which emphasised its positive results rather than its limitations. The headline in the Daily Express that “Aspirin stops bowel cancer” is misleading. The study did not find that aspirin stops bowel cancer, but rather that it may reduce the risk of developing the disease. The Daily Telegraph’s claim that taking a quarter of an aspirin pill every day – “lower than the dose recommended for a child” – can help prevent bowel cancer is confusing. Aspirin should not be taken by children under 16 without specialist advice.
What kind of research was this?
This case-control study looked at the association between regular intake of aspirin at different doses over different lengths of time and the risk of having bowel cancer. This type of observational study is often used to identify factors that may contribute to a medical condition. It compares people who have that condition (the cases) with people who do not have the condition but are otherwise similar to those who do (the controls). Although this type of research can help identify factors that are associated with risk of disease, it has a number of limitations. Most importantly, it cannot establish whether the factor in question caused the disease to develop or whether the disease caused the factor to be present. In other words, it cannot prove cause and effect. Prospective cohort studies, which look at possible risk factors in different groups of people and follow them for many years, provide stronger evidence.
The researchers say that colorectal cancer is the second most common cause of cancer-related deaths and that there is evidence to suggest that aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) can protect against it. However, it is still unclear what dose of aspirin might be effective in prevention and for how long it would need to be taken. The researchers set out to answer these specific questions.
What did the research involve?
The researchers included 2,279 people with bowel cancer and 2,907 healthy people, drawn from a large Scottish case-control study. People with cancer were recruited within 2–3 months of diagnosis, while those without cancer were randomly drawn from a population register. The cases and controls were matched for age, gender and residential area. Those with cancer had their tumours assigned to a specific stage, using information from hospital and primary care staff and departments.
All participants were asked to complete a questionnaire with questions about lifestyle choices and medication use. Medical history, physical activity, smoking status, height, weight and waist circumference were also recorded, as were socio-economic data. Participants recorded their intake of aspirin and other NSAIDs and painkillers. They also filled in a validated food frequency questionnaire. Researchers also recorded data on deaths in the two groups.
The researchers used standard statistical methods to analyse the association between use of aspirin and other NSAIDs and the risk of bowel cancer over five years. They took into account factors that could affect the results (confounders), such as diet, physical activity and other lifestyle choices, and family history. They also looked for any effect of aspirin use on survival rates among the group diagnosed with bowel cancer.
What were the basic results?
Overall, 354 people with bowel cancer (15.5%) were taking low-dose aspirin, compared with 526 in the healthy group (18.1%). The main findings were as follows:
- Low-dose aspirin use (75mg a day) was associated with a 22% reduced risk of colorectal cancer (odds ratio 0.78, 95% confidence interval 0.65 to 0.92).
- The risk reduction was not significant after one year of use, but was significant after five years of regular use. There was no significant risk reduction with more than 10 years of use.
- Use of any NSAID for more than three years was also associated with a significantly reduced risk.
- There was no relationship between taking aspirin or other NSAIDs and survival rates in the group with bowel cancer.
How did the researchers interpret the results?
The researchers say that this is the first study to demonstrate that the lowest dose of aspirin (75mg a day) has a protective effect against colorectal cancer. The effect is apparent as early as one year, but increases with time up to ten years. The researchers say the results are consistent with previous studies demonstrating aspirin’s protective effect.
This large study suggests that taking low-dose aspirin (75mg daily) can reduce the risk of bowel cancer in the population as a whole (rather than in high-risk individuals alone) after five years’ use. Its results are important, but this type of study has limitations that mean it cannot prove cause and effect. Further research is required to confirm or refute these findings and larger prospective studies are needed to establish whether low-dose aspirin does protect against bowel cancer. Some of the study’s limitations are highlighted below:
- It relied on questionnaires filled in by participants, who were asked to accurately recall their lifestyles, medical history and medication use for more than ten years in the past. This could have introduced error or bias into the study if those with bowel cancer remembered their medication use differently to those without the disease.
- It is possible that other factors (confounders) may have affected the results, even though the researchers adjusted their results to take some of these into account.
- It is possible that those diagnosed with cancer may have been taking less aspirin due to early symptoms before diagnosis.
- The study relied on a control group of people who had not been diagnosed with bowel cancer at the time they participated, but who may have gone on to develop the disease later.
Aspirin and other NSAIDs can have gastro-intestinal complications, including ulcers and internal bleeding. They should not be taken regularly without the advice of a doctor.