Experts are now calling for government analysis and advice on the use of aspirin to prevent cancer.
The studies, by one of the world's top aspirin researchers, Professor Peter Rothwell of the Nuffield Department of Clinical Neurosciences at Oxford University in the UK, and colleagues, are published on Wednesday, two in The Lancet and one in The Lancet Oncology.
Rothwell and his team had previously published evidence that suggests a daily low dose of aspirin can reduce the long-term risk of dying from cancer, and that this benefit does not appear until about 8 to 10 years after starting to do so.
But, as Rothwell explained to the press:
"What we have now shown is that aspirin also has short-term effects, which are manifest after only 2-3 years."
He also spoke of the other important finding, about aspirin's effect on reducing metastasis, where cancer begins to spread to other organs:
"In particular, we show that aspirin reduces the likelihood that cancers will spread to distant organs by about 40-50%," said Rothwell, explaining this was just as important a finding because it is metastasis that commonly kills people with cancer.
"No drug has been shown before to prevent distant metastasis and so these findings should focus future research on this crucial aspect of treatment of patients with cancer that hasn't already spread," he added.
This means patients who have already received a cancer diagnosis could benefit from taking aspirin following their diagnosis, so long as the cancer has not already spread.
In their reports Rothwell and colleagues emphasize the urgent need for new trials to confirm these benefits.
For taking daily aspirin is not without risks. There is an increased chance of internal bleeding, particularly in the stomach.
But even in this area, Rothwell and colleagues reveal two new findings that suggest the harms may be less severe than previously thought. One is that the risk of stomach bleeds appears to reduce with prolonged use, and that the risk of dying from a stomach bleed is no greater with aspirin than with a placebo.
Aspirin is one of the world's most widely used medications.
Rothwell explained that in previous studies, when balancing risks and benefits, researchers have tended simply to count "crude numbers" of outcomes such as bleeds, and have not taken into account their severity or development over time.
For their analysis, Rothwell and colleagues also included studies on using aspirin to prevent heart disease and stroke.
They suggest, in absolute terms, and particularly with prolonged use, the risk-reducing benefits of daily aspirin are larger for cancer than for heart disease and stroke.
For the first study, Rothwell and colleagues pooled data from 51 randomized clinical trials that set out to compare the effect of taking a daily dose of aspirin against not taking any aspirin on cardiovascular events such as heart attack and stroke. These trials had also collected data on cancer.
From the results they established that aspirin reduced the risk of cancer death by 15%, and this reduction increased with prolonged use. For instance, for those who took it for 5 years or more, the reduced risk of death from cancer was 37%.
Rothwell and colleagues also found that taking a low dose of aspirin every day did more than reduce deaths from cancer: it reduced incidence. After three years of aspirin use, the reduction in cancer incidence was 23% in men and 25% in women.
In their second study, Rothwell and colleagues examined the effect of aspirin use on metastasis, where cancer spreads from the original site to other parts of the body. For this they pooled data from 5 large randomized UK trials that investigated aspirin use in preventing heart disease and added further information from cancer registers and death certification systems.
The results showed that aspirin use led to a fall in the risk of cancer spreading to other parts of the body. The reduced risk was 36% over the 6.5 years average duration of the studies, and did not vary by age or gender.
In their third study, the one published in The Lancet Oncology, Rothwell and colleagues again examined the efffect of aspirin on cancer risk, but this time they systematically reviewed observational studies (the sort that follow one group of people over time) rather than clinical trials where participants are randomly assigned to different treatment groups.
This analysis confirmed the results they had found from the randomized trial analyses: the reductions in risk were similar.
Some cancers appear to respond more than others to aspirin use. Rothwell said in terms of preventing spread, their findings suggest the largest effect is in adenocarcinomas, which includes cancers of the gut, and colorectal cancer in particular. It also includes most breast and prostate cancers, and some lung cancers.
"In terms of preventing the longer-term development of new cancers, the largest reductions are seen in risk of colorectal cancer and oesophageal cancer, with smaller effects on several other common cancers," said Rothwell.
Cancer Research UK (CRUK) says they are encouraged by the new studies, saying they help to clarify the picture, but this does not mean it is safe to start advising people to take a daily low dose of aspirin to help prevent cancer.
They emphasize that aspirin is not a harmless drug, and for some people the side effect of internal bleeding can be very serious. There are other risks too, such as when people stop taking daily aspirin, they are more likely to have a stroke.
Plus there are potential conflicts with other drugs. For instance, one effect aspirin has is it helps prevent blood clotting by inhibiting the biosynthesis of thromboxane, a blood-clotting prostaglandin released by blood platelets. So people on anti-coagulants or blood thinners should not take aspirin, unless directed by a doctor.
CRUK suggests we need answers to many questions before we can safely advise use of aspirin for cancer prevention. For instance:
- What is the optimal period of time to be taking aspirin for?
- At what age does the biggest benefit and smallest risk occur?
- Who is most likely to benefit, and who is most likely to get side effects?
- How can we minimize the risk of a stroke when people stop taking the drug?
The charity's Chief Medical Officer, Professor Peter Johnson, said on Wednesday that anyone considering taking aspirin to prevent cancer should talk to their GP about whether it is safe for them to do so.
If you do get the go ahead from your doctor then make sure you don't take aspirin on an empty stomach, says CRUK.