Men and women aged 50 and over should consider taking aspirin to reduce their risk of heart attacks and strokes, experts have suggested.
Some people should not take aspirin because of side effects
A 25-year study of 2,500 men, to appear in the British Medical Journal, found the middle-aged are at high enough risk to benefit from daily doses.
Author Professor Peter Elwood, of Cardiff University, called for wider use of the drug.
But the British Heart Foundation warned it might be harmful for some people.
Professor Jeremy Pearson, associate medical director of the British Heart Foundation, said: "Taking a daily dose of aspirin - already routinely prescribed for people who have had a heart attack - is likely to be beneficial for many people over the age of 50.
"The risks and the costs are low, and the potential benefit is significant.
"Whether this approach could prevent as many as one in four heart attacks is yet to be proven, but we look forward to seeing the published results of this study.
"It is important, however, that people are aware of the risks before making the choice to take aspirin long-term.
"The drug can occasionally cause bleeding problems or an allergic reaction, so people should always check with their GP before beginning such a regime."
Doctors already recommend aspirin for patients who have a higher than normal chance of having a heart attack or stroke, as long as there are no other medical reasons not to prescribe the drug, such as unwanted side effects.
Professor Elwood's research found that 80% of men pass this risk threshold by the age of 50, suggesting many more people not already on aspirin might benefit from one 75mg tablet per day.
Speaking at a conference in London held by the Aspirin Foundation, he said: "If everybody were to follow this advice, potentially it could save tens of thousands of lives every year - and at almost no cost."
He said that it was cheap, costing about 1p per day on prescription or 4p a day without a prescription, and safe for most people.
About two to three people in every 1,000 can experience complications such as stomach bleeding, he said.
He believes people should make their own evaluation and decision about whether to take daily aspirin or not, weighing up the risks and benefits.
One British adult dies from heart disease every three minutes - and stroke is the country's third biggest killer, claiming 70,000 lives each year.
The Government wants to reducing the death rate from coronary heart disease and stroke and related diseases in people under 75 by at least 40% by 2010.