May 10, 2007
Oxford University researchers said taking a dose of 300mg a day for five years offered the protection.
Long-term aspirin use is generally not backed because of the risk of stomach problems, but the team said it could benefit those at high-risk of cancer.
Cancer campaigners welcomed the study - published in the Lancet - which examined data from previous research covering more than 7,500 people.
Each year, around 35,000 men and women in the UK are diagnosed with bowel cancer, and more than 16,000 die from the disease.
The researchers looked at the results from two large-scale UK trials carried out in the late 1970s and early 1980s covering over 7,500 people which had given some people 300, 500 or 1,200mg per day doses of aspirin or a dummy pill for five to seven years.
The people in the study were then followed for up to 20 years, and deaths from bowel cancer were logged.
The team say it takes at least 10 years to see an effect, because that is how long it takes for a pre-cancerous growths to develop into cancer.
Other studies have examined aspirin's benefits.
However, this was the first where people were allocated to take different doses of the drug, or the fake version, rather than looking back at the aspirin use of bowel cancer patients and comparing them with healthy people.
High risk group 'benefit'
The researchers found that taking 300mg of aspirin - the equivalent of one pill - a day for five years reduced the incidence of bowel cancer by 74% in the subsequent 10 to 15 years.
An accompanying review of 30 observational studies suggested taking medium-to-high doses of aspirin for 10 years or more reduced the risk of developing the disease by 50%-70%.
The protective effect of aspirin appeared to be consistent regardless of age, sex, race or country of origin.
It was also seen in individuals who had a close family relative with bowel cancer, which normally raises the lifetime risk two to four times.
Professor Peter Rothwell, who led the research, said: "What we are saying is that for a subgroup of people at high risk of bowel cancer, they probably aren't going to be dramatically harmed by taking aspirin.
"They may have some risk of bleeding in the stomach, but they will see potentially significant benefits in the reduction of their colon cancer risk.
"In that situation, I think taking aspirin daily would be worthwhile."
Dr Andrew Chan, from Massachusetts General Hospital in Boston, writing in the Lancet, said the findings were "convincing evidence" that aspirin can reduce the incidence of bowel cancer.
But he added: "With the concerns about the potential risks of long-term aspirin use and the availability of alternative prevention strategies - such as screening - these findings are not sufficient to warrant a recommendation for the general population to use aspirin for cancer prevention."
Rob Glynne-Jones, chief medical advisor to the charity Bowel Cancer UK, said: "This is very interesting and it certainly confirms what we already know and believe about the links between aspirin, polyps and the development of bowel cancer.
"However, these findings are not an indication that the general population should rush out and take what is a high dosage of aspirin on a daily basis.
"Aspirin at this dose over a prolonged period of time could lead to potentially serious side effects in a few people."