"Aspirin could help beat cancer: Daily pill can 'cut odds of dying of breast, bowel and prostate cancer by a fifth'," the Daily Mail reports.
A review of previous studies suggests low-dose aspirin could play a useful role in treating some cancers.
The review looked at 47 studies and attempted to combine the results, looking for evidence of a beneficial effect of low-dose aspirin (which is usually defined as 75-300mg per day) on risk of death in people already diagnosed with cancer.
Significant results were a 24% reduction in risk of death from colon cancer, and possibly an 11% reduced risk of death from prostate cancer. Despite widespread media reports, aspirin was not found to reduce the risk of death from breast cancer.
These results should be viewed with some caution as several studies were omitted from the pooled analyses as they gave conflicting results. In addition, the studies that were included in the analysis were observational, so they cannot show cause and effect.
This means the overall link may not be so clear cut and more good quality evidence is needed. As the researchers rightly conclude, further, rigorous trials need to be carried out to ensure any benefits of aspirin for people with cancer outweigh the risks.
Where did the story come from?
The study was carried out by researchers from Cardiff University and the University of Cambridge, with no external funding, and was published in the peer-reviewed medical journal PLOS One. This is an open-access journal, so you can read the study for free online.
The study was widely reported by the UK media. The quality of that reporting was decidedly patchy in some quarters.
Several media sources incorrectly reported that aspirin boosts breast cancer survival, which was not found in this study. Also, there wasn't sufficient evidence to say there were "signs that aspirin might work against almost all tumours," as reported by The Times.
The Mail's suggestion that aspirin boosts survival for people with kidney and oesophageal tumours was also inaccurate – the results showed no change in risk of death for these cancers with aspirin use.
The Daily Mirror reports that one of the concerns with aspirin use is the risk of bleeding, but quoted the lead author as saying: "we specifically looked at the available evidence of bleeding and we wrote to all authors asking for further data. In no study was serious or life-threatening bleeding reported." While this is technically true, the Mirror failed to point out that only two trials including 799 people on aspirin had available data on bleeding risk. The authors of 21 trials reported that data on bleeding risk was not recorded and the other authors did not respond to the request for information.
What kind of research was this?
This was a systematic review of studies looking at the effect of aspirin taken by people with a diagnosis of cancer. Some of the studies' results were pooled in a meta-analysis. This systematic review included randomised controlled trials (RCTs) (the gold standard) but also observational studies, which are unable to prove cause and effect.
There was a wide variation in these study types and cancers studied, which is known as heterogeneity. A meta-analysis with a high degree of heterogeneity can lead to inaccurate or misleading results.
What did the research involve?
The researchers searched two medical databases, Medline and Embase, for studies of aspirin taken by people with cancer. They identified four RCTs and one review pooling the results of five trials, and 42 observational studies, including large cohort studies.
The studies were grouped according to the type of cancer. Statistical methods were used to work out whether the studies were similar enough to pool or whether they were too different to provide meaningful results.
They also contacted all of the lead authors of the studies requesting data on bleeding risk, as this was only available in two of the published studies.
What were the basic results?
In people diagnosed with cancer, aspirin use was associated with a reduced risk of death from colon and possibly prostate cancer, but not breast or any other type of cancer.
A 24% reduction in risk of death was found from pooling 11 observational studies (hazard ratio [HR] 0.76, 95% confidence interval [CI] 0.66 to 0.88). There were some differences between studies, but they were considered to be similar enough to combine.
However, subset analysis of trials looking at the effect of aspirin for cancers located higher and lower in the colon did not find any difference in risk of death with aspirin. One review of five RCTs found a reduced risk of death from colon cancer, though the researchers were not confident in the reliability of this result. Another small RCT of 57 people did not find that aspirin improved survival.
An 11% risk reduction of death from prostate cancer was found with aspirin use after combining the results of eight similar observational studies (HR 0.89, 95%; CI 0.79 to 0.99). There was no risk reduction if all nine of the studies were combined (HR 0.94, 95% CI 0.76 to 1.17).
Aspirin was found to have no statistically significant effect on breast cancer mortality when combining the results of four similar observational studies (HR 0.87, 95%; CI 0.69 to 1.09).
There was some evidence that aspirin may be effective against cancers with the genetic mutation PIK3CA, but more robust trials were necessary to be confident in the result.
Single observational studies suggested a reduced risk of death for lung, head and neck, and oesophageal cancers plus chronic lymphocytic leukaemia. No difference in risk of death was observed for ovarian, bladder or a mix of female cancers.
How did the researchers interpret the results?
The researchers concluded it is, "likely that low-dose aspirin has a beneficial role as an adjunct treatment of cancer". They say the evidence is strongest for colon cancer and for cancers expressing certain genetic mutations.
The researchers also point out the limitations of the research and the need for large randomised placebo-controlled trials to confirm their suspicions, and recommend that these studies include a variety of different cancer types. In the meantime, they advise people with cancer to discuss the benefits and risks of aspirin with their doctor.
The systematic review looked at 47 studies and attempted to combine the results, looking for evidence of a beneficial effect of low-dose aspirin on risk of death in people already diagnosed with cancer.
The few RCTs identified – the best-quality evidence – did not provide conclusive evidence that aspirin improves survival rates.
The rest of the studies were observational in nature, so cannot prove that aspirin reduces the risk of death from cancer. The only significant results were for a 24% reduction in risk of death from colon cancer, and a possible 11% reduced risk of death from prostate cancer. However, these results should be viewed with some caution, as several studies were omitted from the pooled analyses, as they reported different results. This means that the overall link may not be so clear-cut and more good-quality evidence is needed.
Despite widespread media reports and pictures, aspirin was not found to reduce the risk of death from breast cancer – the results could have occurred by chance.
A variety of aspirin doses were used in the different studies, which makes interpretation of the findings even more difficult.
The researchers attempted to see if aspirin had an effect on the spread of cancer, but combined the results of studies of people with colon, prostate or breast cancer. There are so many variables which cannot be accounted for in this type of analysis that it limits confidence in this result.
Aspirin does reduce the risk of stroke and heart attacks, but can also increase bleeding risk in the stomach and the brain, especially if you have uncontrolled high blood pressure. In this current review, no major bleeding was reported, but data was only available for two small trials out of 47.
No widespread recommendations should be made before we know the possible benefits of aspirin for people with cancer outweigh its risks.
If you are considering taking low-dose aspirin on a daily basis, you should discuss the pros and cons with your GP, or pharmacist, before starting.