A daily low dose of aspirin significantly reduces the number of deaths from a whole range of common cancers, an Oxford University study has found.
The 20% drop in all cancer deaths seen in the study adds new evidence to the debate about whether otherwise healthy people in their 40s and 50s should consider taking a low dose of aspirin each day.
Aspirin is already known to be beneficial for those at high risk of heart disease. But among healthy people, the benefit in lower chances of heart problems only marginally outweighs the small risk of stomach bleeds.
The large size of the effect now seen in preventing cancer deaths may begin to tip the balance in favour of taking aspirin, the scientists suggest, but say that it is a matter for the health bodies who write treatment guidelines.
"These results do not mean that all adults should immediately start taking aspirin," cautions Professor Peter Rothwell of the Department of Clinical Neurology at Oxford University, who led the work. "But they do demonstrate major new benefits that have not previously been factored into guideline recommendations."
"Previous guidelines have rightly cautioned that in healthy middle aged people the small risk of bleeding on aspirin partly offsets the benefit from prevention of strokes and heart attacks, but the reductions in deaths due to several common cancers will now alter this balance for many people."
However, he adds: "I don't think it's necessarily right for the person who did the research to say what guidelines should be. We can't say with absolute certainty that there won't be some unknown harm in taking aspirin for 30 years, but it looks as if there would be pretty large benefits in reducing cancer deaths. People have to accept there's some uncertainty here."
Professor Rothwell and colleagues recently established that a low dose of aspirin (75 mg per day, or a quarter of the normal dose taken for pain relief) taken for longer than five years reduces death rates from bowel cancer by more than a third.
In this new work, scientists from Oxford, Edinburgh, London and Japan used data on over 670 deaths from cancer in a range of randomised trials involving over 25,000 people. These trials compared daily use of aspirin against no aspirin and were done originally to look for any preventative effect against heart disease.
The results, published in the Lancet, showed that aspirin reduced death due to any cancer by around 20% during the trials. But the benefits of aspirin only became apparent after taking the drug for 5 years or more, suggesting aspirin works by slowing or preventing the early stages of the disease so that the effect is only seen much later.
After 5 years of taking aspirin, the data from patients in the trials showed that death rates were 34% less for all cancers and as much as 54% less for gastrointestinal cancers, such as oesophagus, stomach, bowel, pancreas and liver cancers.
The researchers also wanted to determine if the benefits from aspirin continued over time. By using cancer registries and death records, they were able to follow up what had happened to participants in three of the trials.
They showed that risk of cancer death over a period of 20 years remained 20% lower for all solid cancers among those who had taken aspirin (even though they would have been unlikely to have continued taking aspirin after the trials finished), and 35% lower for gastrointestinal cancers.
It took about 5 years to see a benefit in taking aspirin for oesophagus, pancreatic, brain, and lung cancer; about 10 years for stomach and bowel cancer; and about 15 years for prostate cancer. The 20-year risk of death was reduced by about 10% for prostate cancer, 30% for lung cancer, 40% for bowel cancer and 60% for oesophagus cancer.
As the evidence points to a delayed preventative effect against cancer, Professor Rothwell believes that it would be those who started taking aspirin in their late 40s or 50s -- ie before people's risk of cancer starts increasing -- and then continued for 20 to 30 years who might eventually see the most benefit.
Professor Rothwell estimates that in terms of cost-effectiveness, taking low-dose aspirin daily is likely to be much more cost-effective than those interventions already used for preventing cancer, such as screening for breast or prostate cancer.
He does note that more research is necessary to understand more about the effect aspirin has on cancer.
While this study looked at how aspirin affected deaths from cancer, Professor Rothwell and colleagues now aim to look at any protective effect of aspirin on the incidence or progression of cancer. The researchers also point out that more trial data are needed on breast cancer and other cancers that particularly affect women.
"Perhaps the most important finding for the longer term is the proof of principle that cancers can be prevented by simple compounds like aspirin," says Professor Rothwell.
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