NICK GRIMM: One of the biggest ever studies into the positive effects of aspirin has attracted far more participants than researchers expected.
In fact so much so, it's being described as a "mega-trial".
Pat McGrath reports.
PAT MCGRATH: For years, aspirin has been promoted as not only a cure for pain, but a safeguard against future disease.
Late last year, the ABC's Norman Swan told listeners to his program, the Heath Report on RN, that he takes a small dose of aspirin every day.
NORMAN SWAN: Why I'm taking it is I've been reasonably convinced by the evidence that daily low-dose aspirin is quite good at lowering your risk of cancer - particularly bowel cancer.
And I have a bit of bowel cancer in my family.
PAT MCGRATH: Norman Swan says aspirin has proven benefits for people who've already suffered a stroke or heart attack, but he hasn't seen enough evidence that to prove it can prevent coronary disease.
NORMAN SWAN: I am not convinced by the evidence that, if you're otherwise healthy and don't have particular risk factors for heart disease, that aspirin, a daily small dose of aspirin prevents further heart disease or arterial disease.
PAT MCGRATH: Now an international study is trying to offer up some evidence.
More than five years ago, Monash University teamed up with the US-based Berman Center for Outcomes and Clinical Research to begin a clinic trial of aspirin use.
It's so far attracted $50 million in funding, and has been recruiting participants aged over 70 through their GPs.
And it's lead researcher Professor John McNeil from Monash University says the study has just reached a big milestone.
JOHN MCNEIL: Sixteen-and-a-half-thousand in Australia and 2,500 in the US, which makes a total of 19,000, which makes it certainly one of the mega-trials conducted in medicine over the last few years. Many clinical trials that are published in medicine have less than 1,000 - many, less than 100.
PAT MCGRATH: This trial needs a large number of participants because it testing out whether aspirin can prevent illnesses that only a small number of people are likely to develop.
JOHN MCNEIL: If they do this, will this mean that after a few years they remain healthier, they don't have the same risk of being admitted to a nursing home or not being able to look after themselves.
And, that's what we call disability-free survival. Survival, free of cognitive disability or free of physical disability, and if this trial turns out to be positive, we'll have shown that aspirin can actually do this.
But on the other hand, we may equally show that aspirin's side effects just simply don't make it worthwhile.
PAT MCGRATH: And those side-effects can be serious.
JOHN MCNEIL: The big risk is bleeding. When you stop the plug platelets sticking together, there's a tendency to bleed, and sometimes this can be very minor, but sometimes it can happen in more significant parts of the body, and cause real problems.
PAT MCGRATH: John McNeil says the study will begin producing results by 2017.
NICK GRIMM: Pat McGrath.