UK researchers who reviewed pooled clinical trial data covering 95,000 people concluded that there was no net benefit to healthy people taking an aspirin a day as a way to protect against vascular disease although there was for people with existing cardiovascular disease since they were already at much higher risk of another serious vascular event, such as a heart attack, stroke and even death.

The study was the work of researchers at the Clinical Trial Service Unit at the University of Oxford and was published online today in The Lancet.

For the meta-analytical study (that is a study that pools data from a group of studies and re-analyzes it as if it had come from one large study), the Oxford team included six primary and secondary prevention trials that had all compared the long term use of aspirin against controls.

The analysis showed that when aspirin was taken as a preventive against cardiovascular disease in healthy people (primary prevention), it reduced the risk of a non-fatal heart attack by around one fifth. Scaled up to a public health statistic this is equivalent to five fewer non-fatal heart attacks every year for every 10,000 patients treated.

However, this modest benefit is more or less wiped out by the increased risk of gastrointestinal bleeds from long term aspirin use. On a public health statistic level this risk equates to three extra gastrointestinal bleeds a year for every 10,000 people treated, and one in three of those bleeds also leads to a stroke.

In the case of secondary prevention, however, the case for taking the drug is much stronger. Again, as in primary prevention the benefit was a one fifth reduction in the risk of a (further) serious vascular event, but this reduction applies to a risk that is already much higher, resulting in 150 fewer such events a year per 10,000 patients treated. Thus the benefit in this case clearly outweighs the risk of bleeding, which is the same as for the healthy patients.

The benefits of taking aspirin were the same for both men and women in both primary prevention and secondary prevention trials said the researchers.

Leading the team was Professor Colin Baigent of the UK Medical Research Council. He said that:

"We don't have good evidence that, for healthy people, the benefits of long-term aspirin exceed the risks by an appropriate margin."

Many doctors recommend their healthy patients who have high risk factors for coronorary heart disease, such as high blood pressure or high cholesterol, take an aspirin a day. This is because previous research on primary prevention trials has led to guidelines that recommend this as a suitable prevention strategy.

But this new study suggests this may not be such a good policy, because healthy people (ie those who do not have an exisiting cardiovascular condition) with high risk factors for coronary heart disease also have a higher than average risk of intestinal bleeding.

As Baigent explained, the primary prevention trials were done a while ago, before statins became widely available.

"Nowadays, primary prevention with statins and other drugs can safely halve the risk of heart attacks and strokes," said Baigent.

He also explained that when you add aspirin to such drugs, then the further reduction in risk of a serious vascular event is half that of the aspirin on its own, which tips the risk-benefit balance even more toward bad news and "has important implications when judging the likely effects of aspirin in practice".

Baigent and colleagues concluded that while aspirin offers clear benefits for people who already have cardiovascular disease, this latest study does not justify the routine use of aspirin in healthy people who have an above average risk of developing coronary heart disease.

Baigent said it was important to think about the potential harm that can come from prescribing aspirin to healthy people:

"Drug safety really matters when making recommendations for tens of millions of healthy people. We don't have good evidence that, for healthy people, the benefits of long-term aspirin exceed the risks by an appropriate margin," he cautioned.

The study was funded by the UK Medical Research Council, British Heart Foundation, Cancer Research UK, and the European Community Biomed Programme.

"Aspirin in the primary and secondary prevention of vascular disease: collaborative meta-analysis of individual participant data from randomised trials."
Antithrombotic Trialists' (ATT) Collaboration.
The Lancet, Volume 373, Issue 9678, Pages 1849 - 1860.
Published online 29 May 2009.
DOI:10.1016/S0140-6736(09)60503-1

Sources:The Lancet, Oxford University.

: Catharine Paddock, PhD