While small doses of aspirin may protect women from stroke, higher amounts raise the risk, suggests a large, long term study in the US.

According to the Nurses' Health Study, women who took one to six aspirin per week had a lower risk of large artery occlusive infarction (a non bleeding stroke) than women who took no aspirin. But women who took 15 or more aspirin tablets per week just about doubled their risk of subarachnoid haemorrhage (a bleeding stroke), compared with those who took no aspirin. The risk was even higher in older women with high blood pressure who took more than 15 tablets a week.

The information was gleaned from questionnaires filled out every two years by the study participants. Over 15 years, the number of strokes in the group was small. By 1994, after 999,231 person years of follow up, there were only 503 strokes.

This may be one reason why the researchers suggest that even though small amounts of aspirin appeared to have some protective effect against ischaemic (non bleeding) stroke the most common type of stroke it is too early to make public health recommendations based simply upon observation. There is still no clear relation between aspirin use and the total stroke risk (Stroke, 1999; 30: 1764-71).