People aged 70 and over should not take low dose aspirin to avoid heart disease because they are at increased risk of nasty side effects, say researchers.
Aspirin can cause unwanted bleeding
At this age, heart benefits gained were offset by increased cases of serious bleeding, the Australian team found.
The bmj.com study used a model to simulate implications for 20,000 people aged 70-74 with no heart disease.
The British Heart Foundation said aspirin was useful in people at very high risk of heart attacks and stroke.
It said there was good evidence suggesting that, although more research was needed in this population.
Professor Peter Weissberg, Medical Director of the BHF, said: "We support the researchers' conclusion that a properly controlled clinical trial specifically designed for older patients should be undertaken before aspirin is advocated for primary prevention of heart disease in the elderly community."
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.
Aspirin has been linked to an increased chance of bleeding in both the stomach and brain.
The figures thrown up by the model used by the researchers at the University of Tasmania indicated that the overall balance of harm and benefit for taking aspirin could tip either way.
Dr Mark Nelson and colleagues recommended a clinical trial to establish the true benefit or harm of aspirin in elderly people.
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.