An aspirin a day will not help you live longer or prevent your first heart attack, but will increase risk of bleeding in older people, a major study has found.

A daily aspirin will not lower the risk of death or heart attacks in healthy older people but will increase the risk of internal bleeding, a major Australian-American study has found.

The landmark study involving more than 19,000 people aged over 70 has shown that taking a low dose (100mg) of aspirin every day will not prolong life or prevent disability, despite common beliefs the pill helps healthy seniors live longer.

Three papers published in the New England Journal of Medicine on Monday revealed the result of a seven-year study that could lead to a rethink of global guidelines relating to the use of aspirin to prevent common conditions associated with ageing.

The clinical trial did not include people who take aspirin for medical reasons as advised by their doctor, including those who have already suffered a heart attack or stroke.

Instead, it focused on answering the question of whether the drug reduced the risk of healthy seniors suffering their first heart attack or stroke, or losing their good health.

Lead author Professor John McNeil says the results are clear: "If you don't need it, don't take it."

The randomised double-blind trial, called ASPREE, was conducted by Monash University in Australia and the Berman Centre for Outcomes and Clinical Research in the United States.

Using the help of GPs, researchers recruited 16,703 older people in Australian and 2411 in the United States, with approximately 9500 people in both the aspirin and the placebo group.

The participants took a daily low-dose of aspirin every day for nearly five years, with researchers monitoring their health and any occurrences of disease, disability or death.

They found an increase in the number of cases of serious internal bleeding among the aspirin takers (3.8 per cent) compared to the placebo group (2.8 per cent).

"There was more bleeding, particularly from the stomach and upper gastrointestinal tract," Prof McNeil told AAP.

The study also showed slightly higher rates of death in those taking aspirin, primarily from cancer, although the results were not statistically significant, he said.

The "tentative finding" required further investigation as researchers in other studies suggest aspirin may prevent cancer, he said.

Prof McNeil cautioned the findings did not apply to those with existing conditions where aspirin is recommended as a preventative measure against further heart attacks, strokes or angina.

He said anyone with questions should follow the advice of their doctor.

The APREE trial was funded by the US National Institutes of Health, the Australian National Health and Medical Council, Victorian Cancer Agency and Monash University.