Low-dose aspirin, even if not taken daily, may reduce a woman's risk of colon cancer over the long term, according to a new study that did not find the same effect for other types of cancer.
The apparent benefit came at a cost, as women taking aspirin also had higher rates of stomach bleeding and ulcers.
But researchers found that women who took 100 milligrams (mg) of aspirin every other day for at least 10 years ended up with about a 20 percent lower risk of colon cancer after some 18 years of follow up, compared to women who took a placebo.
"We were quite surprised to see it actually," said Nancy Cook, the study's lead author from Boston's Brigham and Women's Hospital, because some researchers had believed that 100 mg of aspirin every other day was too little and too infrequent to matter.
The new study, published in the Annals of Internal Medicine, is a follow-up to an earlier report that found no difference in colon cancer risk among the women during the first 10 years of taking aspirin.
According to the American Cancer Society (ACS), colon and rectal cancers are the third most common types of cancer in the U.S.
Previous studies have found that higher and more frequent doses of aspirin - often taken by people with heart disease - are linked to a reduced risk of death from a number of cancers.
Cook said other studies have also found a delay of about five to 10 years in observing the benefits of regular aspirin use on cancer risk.
The new findings are based on a large study of women looking at the benefits and risks of taking aspirin and vitamin E for prevention of cardiovascular disease and cancer.
In an earlier report on the long term study, about 40,000 women were randomly assigned to take low-dose aspirin or a placebo every other day starting between 1993 and 1996. They were followed through 2004 to see if any differences in heart disease or cancer emerged.
For the new report, about 34,000 of the women from that original group agreed to continue taking yearly surveys through March 2012. They no longer received aspirin or placebo pills, though some continued taking aspirin on their own.
Cook and her colleagues found that women who took low-dose aspirin every other day during the original study period of about 10 years had a colorectal cancer incidence of 11 cases per 1,000 people over 16 years, compared to 14 cases per 1,000 people who took a placebo.
Women who continued to take aspirin after the original trial had the greatest reduced risk, according to the researchers.
There was no effect, however, on risk for cancers of the breast or lung.
"The new results from the Women's Health Study provide important evidence confirming that low-dose aspirin use does indeed lower risk of colorectal cancer, but that this benefit does not ‘kick in' until about 10 years after the start of regular use," Eric Jacobs, strategic director of pharmacoepidemiology at the ACS in Atlanta, wrote in an email to Reuters Health.
Jacobs, who has researched aspirin use and the risk of cancer but wasn't involved in the new study, cautioned that there are known side effects of taking aspirin.
"It is important to remember that aspirin is a real drug with real side effects, including sometimes causing serious, even occasionally fatal, stomach bleeding, even at low doses," he wrote.
"Aspirin use is recommended for most people who have had a heart attack, and has some benefits for colorectal cancer as well, but at this point the American Cancer Society does not recommend that people use aspirin specifically to prevent cancer," Jacobs noted.
Cook and her colleagues found that women who took aspirin were 14 percent more likely to have stomach bleeding and 17 percent more likely to have holes in the lining of their stomach known as peptic ulcers.
Still, she said, "the benefits may outweigh those (risks) because you would be preventing more heart attacks and strokes," adding that the decision to take aspirin should be based on an individual's specific risks.