Should You Be Taking an Aspirin a Day?

Summarized by Robert W. Griffith, MD
February 27, 2006

Introduction

Many men and women take a daily aspirin tablet in the expectation that it will help prevent them having a heart attack. The studies on which this hope was based were nearly all done in men, and the benefits found were assumed to be valid for women, too. However, there are few data concerning women's health after starting an aspirin regimen, so the present study is timely. Here's a summary of the findings that were originally reported in the Journal of the American Medical Association.

What was done

Harvard researchers used a large database to find controlled clinical studies on the use of aspirin in the primary prevention of heart attack, stroke, and other cardiovascular events. (Primary prevention is the activity undertaken to prevent the first occurrence of a condition, e.g. a regular exercise program in midlife to prevent having a first heart attack at 60. Secondary prevention refers to actions taken to prevent recurrence of a previous condition, or worsening of a current condition.) Six studies including almost 100,000 individuals were found to be suitable. Three included only men, one included only women, and two included both sexes.
The 'endpoints' studied were cardiovascular events - death from a cardiovascular disease (which includes stroke), non-fatal heart attack (myocardial infarction, or MI), or a non-fatal stroke. In addition, all-cause deaths and major bleeding episodes were counted.

What was reported

The daily aspirin doses in the six studies evaluated ranged from a low of 50 mg (100 mg on alternated days) to 500 mg a day. The average duration of follow-up was 6.4 years.

Among 51,000 women, regular aspirin intake was linked to a 12% reduction in cardiovascular events. There was a 17% reduction in stroke, with a 24% reduction in ischemic stroke¹, but no significant effects on heart attack or cardiac death.

In 44,000 men there was a similar overall reduction (14%) in cardiovascular events. There was a 32% reduction in heart attack, but no significant effect on stroke or cardiac death.

Aspirin increased the risk of bleeding by 68% in women and 72% in men. Hemorrhage was mostly detected in the gastrointestinal tract (stomach or intestines).

What these results mean

The findings don't explain the difference between women and men in the nature of the protective effect of aspirin. The authors of the study offer several possible reasons, quoting differences in the metabolism and pharmacologic effect of aspirin, and in aspirin resistance. And, of course, the relative frequency of stroke and heart attack differs between women and men, which can easily influence the perceived results of treatment.

It may be misleading to look at the percentage reduction in events without considering that such events are relatively uncommon. A 'number needed to treat' analysis (NNT) is called for. The actual benefit of aspirin in women, based on such an analysis, can be expressed as the prevention of approximately 2 strokes per 1,000 women treated for 6.4 years. In men, there was prevention of approximately 8 heart attacks per 1,000 treated for 6.4 years. (The higher number for men/heart attack is because heart attacks in men are more frequent than strokes in women.)

In the same way, a 'number needed to harm' analysis (NNH) can be revealing. It was estimated that aspirin therapy for 6.4 years resulted in an additional 2.5 hemorrhagic events per 1,000 women and 3 such events per 1,000 men.

Using the NNT and NNH figures, it might be argued that aspirin therapy is more harmful than beneficial in women, but more beneficial than harmful in men. This is rather simplistic. Don't try to answer the question in the title of this article for yourself, based alone on what you read here or elsewhere on the Internet. Better by far to be guided by your family physician, who knows you, your body, your family history, and your lifestyle. She or he can advise you about embarking on such a long-term path.

Source

• Aspirin for primary prevention of cardiovascular events in women and men. A sex-specific meta-analysis of randomized controlled trials. JS. Berger, MC. Roncaglioni, F. Avanzini,  et al., JAMA, 2006, vol. 295, pp. 306--313

Footnotes
1. Ischemic stroke is the most common type of stroke - accounting for almost 80% of all strokes - and is caused by a clot or other blockage within an artery supplying brain tissue. Hemorrhagic stroke - bleeding from a brain blood vessel - accounts for 10% to 15% of strokes.

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