An Aspirin a Day Keeps the Doctor Away, Doesn't it?

Summarized by Robert W. Griffith, MD
August 1, 2002 (Reviewed: August 20, 2004)

Introduction

For the last 30 years people have been taking small daily doses of aspirin to ward off heart attacks. Is this a sensible thing to do? What's the evidence in favor, or against, the practice? We'll try to explain, using a recent article in the New England Journal of Medicine as a background.

The idea of preventing coronary artery disease, the forerunner of heart attacks (myocardial infarction, or MI) is well founded. Treating high blood pressure, lowering raised cholesterol levels, losing weight, exercising, and stopping smoking are all effective ways of lowering your chances of having an MI. So where does aspirin fit it?

Aspirin has both anti-platelet and anti-inflammatory effects, both useful properties when it comes to reducing the likelihood of coronary thrombosis. Interference with the function of blood platelets is able to stop thrombosis, or clotting, while the anti-inflammatory effect can help prevent development of coronary atherosclerosis, which has recently been associated with inflammatory changes in the cell layer lining the arteries.

The evidence in favor

In the 1970s several studies were reported that strongly suggested that daily aspirin consumption was associated with a reduced risk of fatal or non-fatal MI. These studies examined groups of persons who were not randomized to the treatment, so that the criticism could be made that people who took aspirin were more likely to have been leading a healthier lifestyle.

More recently two large studies have been published, which support the potential beneficial effects of aspirin. The Nurses' Health Study examined the rate of coronary events in over 87,500 women over a 6-year period. Intake of one to six aspirin tablets a week was associated with a reduced rate of MI by 25%. In a Cleveland Clinic Foundation study of more than 6,000 patients with suspected coronary disease, regular aspirin intake was associated with a 33% reduction in the risk of death from any cause.

Some randomized prospective trials¹ have been done. Four out of five listed by Dr Lauer showed reductions in the risk of heart attacks, varying from 4% to 44%. Pooling the results of these four, the average reduction in risk was about 15%.

These studies suggest that the risk of MI in people is reduced by aspirin intake to a greater extent in people already at risk of coronary heart disease because of other risk factors they possess. There may be an exception to this, though -- in the Thrombosis Prevention Trial, the risks of having an MI were almost halved in patients with a low blood pressure (the systolic -- top number - below 130 mmHg), but aspirin didn't have a protective effect in those with a systolic blood pressure above 145 mm Hg.

Other benefits of aspirin?

Some studies have suggested that regular aspirin intake can reduce the risk of developing cancer of the ovary, esophagus (gullet), stomach, pancreas, colon, and rectum. None of these were prospective trials, however, and the results are therefore not so weighty.

And the evidence against?

Almost all studies have shown an increased risk of hemorrhage with aspirin use. This is because of it's interference with the function of platelets. The most common site for bleeding is the gastrointestinal tract -- the stomach, small, or large intestine. Minor hemorrhage may appear as bruising or nosebleeds.

In an analysis of the five randomized trials referred to above together with other trial results, aspirin treatment reduced the risk of coronary events by about 30% per year, while the risk of hemorrhagic stroke was increased by 2% per year, and the risk of gastrointestinal bleeding increased by 6% per year.² Put another way, 67 patients would need to be treated with aspirin for 5 years to prevent one MI, while 88 would need to be treated to prevent an MI without a major bleeding episode occurring amongst them. In patients at increased risk in the first place, 44 patients would need to be treated to avoid one MI, compared with 53 needed to avoid an MI without a major bleeding episode.

So, what to do?

Like many other decisions in preventive medicine, it's a matter of balancing the risks, which is, after all, a form of gambling. To take the best decision, it's important to know the odds. And the odds in favor of aspirin improve as the patient has a greater likelihood of having a coronary event, due to known risk factors.

Dr Lauer's review suggests using your calculated 10-year risk of coronary heart disease, which you can obtain using one of the two methods given in the links below. Both of these are based on results from the well-known Framingham study, which establish a risk scoring system incorporating age, smoking status, total cholesterol level, high-density lipoprotein (HDL) cholesterol level, and systolic blood pressure. The results are given as the percentage risk of having a heart attack in the next 10 years.

If your 10-year risk is 6% or less, you shouldn't take aspirin. If it's over 15%, and you don't have a known allergy to aspirin, or bleed easily, or have a peptic ulcer, you should take aspirin. And if it's in between (7% to 14%), it's up to you to decide (with the help of your doctor). If you have diabetes, heart or kidney damage from high blood pressure, or are not physically fit, you should consider aspirin therapy more seriously.

One baby aspirin (81 mg) a day, taken with food, is enough. There's no advantage in taking higher doses for this particular use, and you may well increase the chance of side effects.

Source

• Aspirin for primary prevention of coronary events. MS. Lauer, N Engl J Med , 2002, vol. 346, pp. 1468--1474

Footnotes
1. In a randomized prospective study, volunteers are assigned to one of two (or more) treatment groups in a random way, and the findings from the two groups are compared at the end of the study.
2. These are 'relative' risks. If the 'absolute' risk of having a coronary event in 5 years is 5%, aspirin reduces the absolute risk by 0.3% each year, but increases the risk of stroke by 0.02%, and of GI bleeding by 0.06% per year.

Related Links
Will You Have a Heart Attack as You Age? You Can Identify and Reduce Your Risk!
What's your risk of having a Heart Attack?

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