OXFORD, UNITED KINGDOM. While there is conclusive evidence that taking aspirin daily reduces the incidence of heart attack, stroke and vascular death among people with existing heart disease or a prior stroke (secondary prevention), there is no convincing evidence that the daily aspirin "ritual" reduces the risk of heart attack and stroke in people without cardiovascular disease (primary prevention). A group of researchers at Oxford University now conclude that aspirin is of questionable value in primary prevention since the risk reduction it confers is small and, to a large extent, is counterbalanced by an increase in hemorrhagic stroke and internal bleeding.

Their study of the results of 6 primary prevention trials found that the yearly incidence of serious vascular events (heart attack, stroke, or vascular death) among 95,000 participants with low risk for cardiovascular disease was 0.54%/year. Among participants who took aspirin daily, the incidence was 0.51%/year as compared to 0.57%/year among those who did not. On the other hand, 0.10%/year of participants on aspirin experienced a major gastrointestinal or intracranial bleed as compared to only 0.07% doing so among non-aspirin users. Of particular interest is the finding that taking aspirin on a daily basis did not reduce the risk of an ischemic or hemorrhagic stroke in participants at low average risk for cardiovascular disease.

There was no indication that the small reduction in vascular events observed among aspirin users was related to age, gender, smoking history, blood pressure, cholesterol level, body mass index, history of diabetes, or predicted risk of coronary heart disease. In particular, there was no significant trend in the proportional effects of aspirin in people at very low, low, moderate, and high estimated risk of heart disease.

Evaluation of the results of 16 secondary prevention trials revealed that daily aspirin consumption reduced the risk of a serious vascular event from 8.2%/year to 6.7%/year with a statistically non-significant increase in the risk of hemorrhagic stroke and major extracranial bleeds.

The Oxford researchers conclude that their observations "do not seem to justify general guidelines advocating the routine use of aspirin in all apparently healthy individuals above a moderate level of risk of coronary heart disease."

Baigent, C, et al. Aspirin in the primary and secondary prevention of vascular disease. The Lancet, Vol. 373, May 30, 2009, pp. 1849-60