Taking aspirin every day could cut your risk of developing cancer, report BBC News and The Daily Telegraph among other news outlets, after the publication of a large-scale review of the evidence.
People aged between 50 and 65 who take aspirin every day for 10 years could cut their risk of bowel cancer by 30% and cancers of the throat and stomach by 25%, according to the study published in the Annals of Oncology.
Aspirin is an antiplatelet, which means it reduces the risk of clots forming in your blood. Platelets may also protect cancer cells in the body, and it has been suggested aspirin's effect on them may hinder this process. However, the exact mechanism is not well understood and more research is needed.
Taking aspirin every day comes with a serious health warning as it can cause serious side effects such as ulcers and bleeding from the stomach, particularly in elderly people.
However, the researchers argue the benefits of taking the drug need to be balanced against the harms.
While the findings of this study show promise, it is not clear whether the methods used in compiling it were systematic, so the results may not be entirely reliable.
Anyone thinking of taking aspirin for prevention should talk to their GP first.
Who may not be able to take aspirin?
Before taking aspirin, including for pain relief, you should talk to your GP or pharmacist if:
you are pregnant, trying for a baby or breastfeeding
you have a blood disorder
you have ever had a stomach ulcer
you suffer from asthma
you have liver or kidney problems
you have high blood pressure
you have haemophilia or any other bleeding disorder
you have had an unusual or allergic-type reaction to any medicine
you are taking other medicines
Aspirin should also not be given to children under 16 years of age
Where did the story come from?
The study was carried out by researchers from a number of institutions across Europe and the US, including Queen Mary University of London.
It was funded by Cancer Research UK, the British Heart Foundation and the American Cancer Society. The study was published in the peer-reviewed medical journal Annals of Oncology.
Several of the study's authors are consultants to, or have other connections with, pharmaceutical companies with an interest in antiplatelet agents such as aspirin.
As might be expected with cancer-related news, the research was widely covered in the press. Most of the coverage was uncritical, although most stories warned of the side effects of taking aspirin.
What kind of research was this?
This was a review of evidence on the association between aspirin and incidence of deaths from cancer and cardiovascular disease, and potential harmful side effects.
It is not clear whether this was a systematic review, where the evidence is rigorously appraised for its quality and risk of bias. The researchers did not carry out a meta-analysis of the results of studies included, but compiled their own estimates.
The authors say regular aspirin is known to reduce the incidence of cardiovascular disease both in the general population and in high-risk groups, although it is currently only recommended for those at high risk.
However, an increasing body of evidence suggests it may also have a role in cancer prevention. Aspirin is also associated with a risk of bleeding and peptic ulcers. The researchers argue the benefits of taking the drug need to be balanced against the harms.
What did the research involve?
Researchers gathered evidence on the effects of aspirin on cancer risk and cancer deaths from systematic reviews published between 2009 and 2012, as well as from some individual studies on specific cancers. Further systematic reviews undertaken by some of the researchers were not included, but were discussed at the "evidence review meeting".
It is not clear how these studies were chosen or whether further studies on the topic were excluded and, if so, what criteria were used to decide which studies to include or exclude.
Evidence for aspirin's effect on cardiovascular disease was taken from one large meta-analysis. The authors based their calculations of the effect aspirin would have on cardiovascular disease by using UK rates from 1998 for cardiovascular-related incidents and deaths, which they adjusted to take account of downward trends in recent years in both the UK and the US.
The researchers used a detailed unpublished analysis of the harmful effects of aspirin.
They calculated the overall benefits and harms for taking aspirin for 10 years, starting at ages 50, 55, 60 and 65, separately for men and women. They made several assumptions in their analysis:
the cardiovascular benefit and adverse effects only occur during active treatment (the 10-year period)
the protection against cancer begins three years after initiating aspirin and continues for an additional five years after stopping aspirin
the protection against cancer mortality begins five years after starting aspirin use and lasts for an additional 10 years after treatment is stopped
the protective effects are seen only in colorectal, oesophageal, gastric, breast, prostate and lung cancers
What were the basic results?
The researchers calculated that for average-risk individuals aged 50 to 65 taking aspirin for 10 years, there would be a relative reduction of between 7% (women) and 9% (men) in the number of cancer, myocardial infarction or stroke events over a 15-year period, and an overall 4% relative reduction in all deaths over a 20-year period.
Below are their calculations of the effect of aspirin in reducing the risk of cancers and cardiovascular events, giving what the researchers say are "conservative" estimates:
colorectal (bowel) cancer – 30% reduction in incidence and 35% reduction in deaths
oesophageal cancer – 25% reduction in incidence and 45% reduction in deaths
gastric cancer – 25% reduction in incidence and 30% reduction in deaths
lung cancer – no reduction in incidence, 10% reduction in deaths
prostate cancer – 5% reduction in incidence, 10% reduction in deaths
breast cancer – 5% reduction in incidence, no reduction in deaths
heart attack – 18% reduction in incidence, 5% reduction in deaths
stroke – 5% reduction in incidence, 21% increase in deaths
Their calculations on the risk of side effects from taking aspirin are:
major (extracranial) bleeding – 70% increase in incidence
gastric bleeding – 70% increase in deaths
peptic ulcer – 70% increase in deaths
They also say the effects are not apparent until at least three years after starting aspirin, and some benefits may be sustained for several years after stopping.
They found no difference between low and high doses of aspirin in terms of health benefits, although there were no studies that did direct comparisons.
How did the researchers interpret the results?
The researchers say once aspirin's effect on cancer risk and mortality is taken into account, the benefits of taking aspirin outweigh the risks.
They calculate that to get any benefit, people need to start taking a daily dose of between 75mg and 325mg for a minimum of five years. Longer use is likely to have greater benefits, they say.
Further research is needed to determine the optimum dose for taking aspirin and duration of use, and to identify those at increased risk of bleeding.
In an accompanying press release, lead author Professor Jack Cuzick of Queen Mary University of London said: "It has long been known that aspirin – one of the cheapest and most common drugs on the market – can protect against certain types of cancer.
"But until our study, where we analysed all the available evidence, it was unclear whether the pros of taking aspirin outweighed the cons.
"Whilst there are some serious side effects that can't be ignored, taking aspirin daily looks to be the most important thing we can do to reduce cancer after stopping smoking and reducing obesity, and will probably be much easier to implement."
While the findings on aspirin and cancer show promise, it is not clear that the results are reliable from the methods reportedly used to compile this review.
This is because it included studies of varying design and quality, with much of the evidence coming from observational studies, which, while useful, cannot be totally relied on to test the effectiveness of healthcare interventions.
It's not clear how the studies included in the review were chosen and whether others on the same topic were excluded. It is also not clear whether or not this was a systematic review, where studies are rigorously appraised for their quality, and criteria are established for their inclusion.
Aspirin can cause major side effects such as peptic ulcers and bleeding from the stomach, particularly in older people. It's important to consult with your GP before deciding to take aspirin regularly.