The risk of disabling or fatal bleeding also climbed with age, coming in at less than 0.5 percent for under 65s and 1.5 percent for those aged 75-84 and almost 2.5 percent for patients aged 85 or over.
He noted that the cost of treating a patient with aspirin and an inhibitor was "very small" in comparison with hospital treatment for a major bleed.
For the studies, in the University of Oxford, Peter Rothwell led a team in analyzing medical records for higher than 3,000 patients who had a stroke or heart attack and daily taken aspirin or its equivalent for many years.
In a statement, Rothwell said that "We have known for some time that aspirin increases the risk of bleeding in elderly patients".
Doctors are now being advised to prescribe PPIs alongside aspirin to reduce the risk.
Proton-pump inhibitors (PPIs) are commonly used to treat heart burn by reducing levels of stomach acid. Studies have shown that low doses of the blood-thinner can significantly lower the risk of heart problems, especially among people who have already had a heart attack or stroke. Many healthy adults also take them in an attempt to prevent illness.
Over a 10-year period, 314 patients were admitted to hospital for bleeding. And for the patients who are under 75 to 84, the rate rises to 3.5 percent.
Although heart attack or stroke risk also increases with age, the researchers said that in the overs 75s major upper gastrointestinal bleeding caused by antiplatelet therapy was at least as likely to be disabling or fatal as recurrent stroke - if a PPI was not co-prescribed.More news: Wolfenstein The New Colossus Revealed
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The study was an observational study, rather than a randomised trial, meaning it's not possible for this study to show that the increased risk is entirely caused by aspirin.
Professor Helen Stokes-Lampard, chair of the Royal College of Global Positioning System, said: "The study does reassure us that in most cases, aspirin is still the most appropriate course of treatment for patients, but highlights the importance of managing its use carefully and effectively and that some patients may require additional medication to protect them".
Millions of pensioners take the blood-thinning pills to ward off heart attacks and strokes.
In a linked comment published in The Lancet, German expert Professor Hans-Christoph Diener, from the University of Duisburg-Essen, wrote: "The first effect of (this) study is that the benefit-risk association in long-term anti-platelet therapy should be evaluated every three to five years in patients older than 75 years".
Half were aged 75 or over at the start of the 10-year study.
Dr Tim Chico, consultant cardiologist at the University of Sheffield, said: "Prescription of any drug is a balance between the benefits of the medication against its risks, and aspirin is no different". A daily aspirin dose of 75 mg is usually prescribed in both the US and Europe to prevent heart attacks or strokes.
"I wouldn't take aspirin as a form of primary prevention".
What about stomach protection for people under-75 given daily aspirin after strokes and heart attacks?.
The consumption of low-dose aspirin on daily basis to reduce the risk of heart disorder or cancer can lead to stomach bleeds and death. Heartburn medication would allow people 75 years and older to keep the precautionary advantages of aspirin while averting its risky side-effects, as was reported in the medical journal the Lancet.