The benefits for women of taking a daily dose of aspirin to stave off serious illness, such as cancer and heart disease, are outweighed by the risks, scientists have concluded.
Previous studies have suggested that 6,000 lives could be saved each year if middle aged adults took aspirin each day.
But new research which studied 30,000 women over 17 years showed that the protective effect was marginal yet the risk of stomach bleeding was high.
Women under 65 should not take a dose of aspirin every day as the dangers outweigh the benefits, a major study has found.
Research on 30,000 healthy women found the advantages of giving regular low doses of aspirin to stave off potentially fatal illnesses - such as cancer and heart disease - are outweighed by the risks.
However, the research by Utrecht University said that women over the age of 65 would benefit from taking the pill daily.
The study, published online by the journal Heart, show the balance begins to shift with increasing age. The researchers found that a blanket treatment "is ineffective or harmful in the majority of women with regard to the combined risk of cardiovascular disease, cancer and major gastrointestinal bleeding."
However the balance begins to shift with increasing age, suggesting women over 65 are likely to still benefit from taking an aspirin a day.
To test whether aspirin was useful participants were randomly assigned to take either 100 mg of aspirin or a dummy tablet every other day, to see whether it curbed their risk of heart disease, stroke, and cancer.
During the trial period, which lasted 10 years, 604 cases of cardiovascular disease, 168 cases of bowel cancer, 1832 cases of other cancers. However there were 302 major gastrointestinal bleeds requiring admission to hospital during the time.
To test whether aspirin was useful participants were randomly assigned to take either 100 mg of aspirin or a dummy tablet every other day, to see whether it curbed their risk of heart disease, stroke, and cancer.
During the trial period, which lasted 10 years, 604 cases of cardiovascular disease, 168 cases of bowel cancer, 1832 cases of other cancers. However there were 302 major gastrointestinal bleeds requiring admission to hospital during the time.
Over the subsequent seven years, a further 107 cases of bowel cancer and 1388 other cancers were diagnosed.
Compared with placebo, regular aspirin was linked to a lower risk of heart disease, stroke, bowel cancer, and in some women, other cancers, but only marginally so.
Earlier this year, more than a million people with a common heart condition - atrial fibrillation - were told not to take aspirin to guard against stroke, in a reversal of previous NHS advice.
The guidance in June from the National Institute for Health and Care Excellence that the pills are ineffective in reducing the danger for those suffering heart rhythm disorders and that the risk of side-effects outweighs their benefits.
