Does an aspirin a day reduce the risk of stroke in women?

(July 2001)

The answer according to news stories based on an article in the journal Stroke by investigators from Harvard Medical School and the Harvard School of Public Health is yes. But is that really true?

The authors followed almost 80,000 nurses ages 30 to 59 years for fourteen years, during which time a total of 503 new strokes occurred, 295 of them due to a clot (called ischemic) and a smaller number due to bleeding (hemorrhagic).

Those who took either one to two or three to six aspirins per week had a slight (13 to 15 percent) reduction in stroke occurrence, but that was not statistically significant. Women who took seven or more per week showed a 17 to 31 percent increase in stroke risk that was statistically significant for those using seven to fourteen aspirins per week, but was not for those taking fifteen or more a week.

When they sub-divided the strokes into two groups, clots and bleeding, there was a strong suggestion that those taking more than fifteen aspirins a week had increased risk of bleeding. There was no evidence that taking aspirin had any effect, favorable or unfavorable, on the risk of strokes due to clots in the blood vessels of the brain. The authors then analyzed three subcategories within strokes due to clots and found that in one of the three sub-categories (clots in large blood vessels), one to two aspirins a week reduced the risk; the benefit of aspirin was less certain for those who took three to six a week. The alleged reduction in risk mainly occurred among women who had high blood pressure and/or smoked.

The authors conclude "women who take one to six aspirins per week have a reduced risk of large artery occlusive infarction (clots in large brain vessels) but those who use fifteen or more aspirins per week have an increased risk of hemorrhage."

Commentary: The bottom line is that aspirin in dosage of one to six a week did not overall protect against strokes due to blood clots in blood vessels of the brain and in higher aspirin dosage (fifteen or more a week) increased the risk of bleeding into the head. That is the real finding. The further analysis of 28 percent of the ischemic strokes, those 82 cases involving clots in large blood vessels, that showed a reduced risk for women taking one to six aspirin a week who had high blood pressure or were smokers, should not be allowed to obscure the overall negative findings.

This study has been over-interpreted in the press. The Reuters story said "healthy women who take one to six aspirin tablets a week may cut their risk of having the most common type of stroke (ischemic stroke)." That is not true. The study showed no benefit for that type of stroke. Only for one relatively small sub-group of ischemic strokes (due to blood clots in large vessels) did there appear to be benefit from aspirin and then only for women who had high blood pressure or who were smokers. And even that alleged benefit may be by chance alone.

The fact is, unlike the public relations and media reports, this study is essentially negative.

There is little reason for a healthy person to take aspirin regularly in the hope of preventing a stroke. On the other hand, the evidence is good that those who have already suffered a minor stroke or have a very irregular heartbeat (that can lead to clots and a stroke) do have a moderately reduced risk of stroke if they take small doses of aspirin. It would be unfortunate if healthy people reacted to the misinformation and hype by taking aspirin in the hope of preventing strokes. Besides, even small amounts of aspirin can occasionally result in bleeding into the head in those who take it several times a week for prolonged periods. Aspirin, like any other drug, should be taken only when there is clear evidence it can be beneficial.

Iso, H., et al. Prospective study of aspirin use and risk of stroke in women. Stroke. Vol 30(9). (September) Pgs 1764-1771. 1999.