Aspirin is well known to reduce the risk of heart attacks and stroke in those who have already suffered these problems.

The pooled results of four major international clinical trials involving over 48,000 people, over 25,000 of whom were treated with aspirin, were subjected to detailed analysis.

The results showed that aspirin cut the risk of first heart attacks by almost a third. But it also increased the risk of bleeding complications by almost 70 per cent.

For treatment, it is therefore important to identify how likely a person is to have a heart attack. If the risk of a heart attack is calculated as being moderately high, at 15 per cent over 10 years -- that is, a one in six or seven chance -- taking 75 mg of aspirin daily is worthwhile, the data indicate.

Except in people with ulcers or high blood pressure that is not being treated, the chance of preventing a heart attack far outweighs any possible harm.

And in people at moderate risk -- between 5 and 15 per cent -- the benefits and disadvantages cancel each other out, so the authors conclude that taking aspirin is probably worthwhile.

But in people at low risk, with a one in 20 -- or 5 per cent -- chance of having a heart attack within the next 10 years, the side effects, such as bleeding from the stomach, are likely to outweigh any benefit.

The difficulty is that people do not often know their true level of heart disease risk, and may simply decide to treat themselves, say the authors. This risk can only be calculated using a set of specially devised tables, taking into account all possible risk factors, such as smoking, family history and cholesterol levels.

(Reference: HEART, Aspirin for primary prevention of coronary heart disease: safety and absolute benefit-related coronary risk derived from meta-analysis of randomised trials 2001;85: 265-71.)

[Contact: Professor Larry Ramsay]

15-Feb-2001