Major new research shows that people at high risk of heart attack or stroke should be given one or two baby aspirins a day, instead of the standard maximum strength adult aspirin.
The study also concluded that the aspirin can help a wider range of people with potential heart trouble.
Aspirin is the cornerstone of blood thinning treatment for people who have had a heart attack or stroke, but is not normally used for those who haven't had a heart attack or stroke but suffer from risky conditions such as diabetes, chest pain, irregular heart beat and diseased leg arteries.
The researchers found that aspirin and antiplatelet therapy reduced the risk of a serious heart attack or stroke by a quarter and of non-fatal heart attack by a third.
It also cut less serious strokes by a quarter.
The findings come from an analysis that combines the evidence accumulated over the years on the effectiveness of aspirin and its alternatives in staving off heart trouble. Coordinated by scientists at Oxford University in England, it encompassed 287 studies involving 212 people.
The most crucial advance offered by the study is in defining the appropriate dose of aspirin for long-term therapy, said Dr. Eric Topol, cardiology chief at the Cleveland Clinic who was not involved in the analysis.
"That's a big thing. Before this analysis we weren't sure what the dose was at all," Topol said. "325 milligrams was readily available, so it was used out of convenience, but now I think we've zeroed in in the range of 80 to 160."
Most doctors and heart specialists use a dose of 325 milligrams of aspirin per day when applying it as a blood thinner. That's the dose in a maximum strength adult aspirin tablet.
The latest analysis shows that between 75 milligrams and 150 milligrams works just as well, with less chance of internal bleeding.
In the United States, baby aspirin, also available as low-dose adult aspirin, contains 81 milligrams.
In Europe aspirin comes in doses of 75 milligrams and 300 milligrams.
"There are two problems," Topol concluded "Doctors are giving too much or they are not giving any at all. We have a lot of work to do now to get all the patients treated and at the right dose."
"That dose should be considered to be one or two baby aspirin and not the standard 325 milligrams," he said.
A similar analysis performed in 1994 by the same group solidified the role of long-term daily aspirin treatment in avoiding second heart attacks or strokes in people who have already had one.
The latest review, published in the British Medical Journal, is a much-anticipated update.
"We've got clear evidence now from this review that people who haven't yet had a heart attack or stroke do benefit, but they are being treated less than half the time with aspirin," said one of the investigators, Dr. Colin Baigent, an epidemiologist at Oxford University.
"If we were to tackle that group, that would save about 40,000 extra lives a year worldwde," he said.
He attributed the under-use of aspirin, one of the world's most popular drugs, to a lack of clear guidelines. Aspirin can also cause stomach problems such as bleeding and ulcers, which could also explain why it is not more widely used.
The study concluded that most healthy people, who have less than a 1 percent chance of having a heart attack or stroke, should not regularly take aspirin to prevent heart trouble.
The key is to determine how unhealthy someone has to be to benefit from aspirin, Baigent said. If the likelihood of having a heart attack or stroke is smaller than the chance of internal bleeding from the aspirin, the drug would cause more harm than good.
People should always consult a doctor before taking the drug regularly.
Professor Sir Charles George, medical director of the British Heart Foundation, which funded the research with the MRC, said the study provided further proof that aspirin had major benefits for people at high-risk of heart attack or stroke.
"Aspirin is not an appropriate treatment for everyone but it is important that all those who might benefit are actually offered it," he said in a statement.
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