Healthy women who take a low to moderate dose of aspirin could reduce their risk of dying early, particularly from heart disease, a study suggests.
Daily aspirin use is controversial
The US work monitored 80,000 women's use of aspirin for over 20 years.
Writing in Archives of Internal Medicine, the researchers said the benefits were associated with taking low or moderate doses of aspirin.
But UK experts said the findings contradicted other studies, and warned aspirin could cause bleeding.
The researchers from Massachusetts General Hospital and Harvard Medical School, Boston, and colleagues monitored the women aged 35 to 60, who were all taking part in the Nurses' Health Study.
They were checked in 1980, and then every two years until 2004.
At the beginning of the study, none of the women had cardiovascular disease or cancer.
Each time they were assessed, the women were asked if they used aspirin regularly and, if so, how many tablets they typically took per week.
Over the course of the study, just under 30,000 took low to moderate doses (one to 14 325 milligram tablets per week), while 5,000 took more than 14 tablets per week.
By June 1, 2004, 9,477 of the women had died - just under 2,000 of heart disease and 4,469 of cancer.
Each death was compared with seven or eight other women in the study of a similar age and situation.
Women who reported taking low to moderate doses of aspirin had a 25% lower risk of death from any cause than women who never used aspirin regularly.
Aspirin users had a 38% lower risk of death from cardiovascular disease, and a 12% lower risk of death from cancer.
Women who took high doses of aspirin did not appear to benefit.
Writing in Archives of Internal Medicine, the researchers led by Dr Andrew Chan, said: "Use of aspirin for one to five years was associated with significant reductions in cardiovascular mortality.
"In contrast, a significant reduction in risk of cancer deaths was not observed until after 10 years of aspirin use.
"The benefit associated with aspirin was confined to low and moderate doses and was significantly greater in older participants and those with more cardiac risk factors."
The team say there are a number of ways in which aspirin could protect people, including preventing inflammation and cell damage from oxygen exposure.
However, they say that because the study looked at women who chose whether or not to take aspirin, rather than a randomly assigned clinical trial, the results do not suggest that all women should take aspirin.
They say more research needs to be done to evaluate aspirin's effects.
'Out of step'
But Dr John Baron, of Dartmouth Medical School in New Hampshire, said the Women's Health Study, which followed 40,000 women for 11 years, found no benefits in taking aspirin.
He added: "Is aspirin really that good or is there some other explanation for the findings that differ so much from those of the WHS and other primary prevention trials?"
And Professor Peter Weissberg, medical director at the British Heart Foundation said: "The Nurses' Health Study, while interesting, is out of step with all the previous studies and should not therefore be used as evidence in favour of all women taking aspirin to prevent heart disease.
"There is good evidence to suggest that people who have had a heart attack, or are considered at risk of heart disease benefit from being prescribed aspirin.
"However, it isn't suitable for everybody to take to prevent heart disease, as regular aspirin use is associated with an established risk of bleeding.
"This will no doubt remain a very controversial issue where we simply do not have a definitive answer."