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Aspirin prevents stroke, but not MI, in women

BMJ 2005; 331 doi: (Published 21 July 2005) Cite this as: BMJ 2005;331:0-f

Question Does aspirin prevent cardiovascular disease in women?

Synopsis This study represents the largest and best evidence to date for women. Women over 45 without a history of coronary artery disease, cerebrovascular disease, or cancer were initially enrolled in a three month placebo run-in to establish compliance with the study protocol. Those who complied throughout the run-in period (n = 39 876) were randomised (allocation not specified, but likely concealed) to receive either 100 mg aspirin every other day or matching placebo. They were followed up for a mean of 10 years, with 97% complete data on morbidity and 99% complete data on mortality. Very impressive. The mean age was 55 years, and the 10 year risk of heart disease was less than 5% in 85% of the women. Groups were balanced at the start of the study, outcomes were blindly assessed, and analysis was by intention to treat. Women taking aspirin were less likely to have a stroke (1.1% v 1.3%; P = 0.04; number needed to treat = 444 for 10 years) or transient ischaemic attack (0.9% v 1.2%; P = 0.01; NNT = 384 for 10 years) than women taking placebo. However, groups did not differ in the likelihood of myocardial infarction (MI) (0.99% for aspirin and 0.97% for placebo) or death from cardiovascular causes (0.60% v 0.63%), any major cardiovascular event (2.4% v 2.6%), or any cause (3.1% v 3.2%). Gastrointestinal bleeds requiring transfusion were more common in the aspirin group (0.64% v 0.46%; P = 0.02; number needed to treat to harm = 553 for 10 years). The study was powered to have an 86% chance to detect a 25% reduction in the primary outcome of any major cardiovascular event. Review of the survival curve reveals a steady but small trend in favour of aspirin regarding the primary outcome. This apparent benefit, equivalent to a 5% to 10% relative reduction in all cause mortality, was not statistically significant despite the study's large size.

Bottom line Aspirin reduces the risk of stroke and transient ischaemic attack in women but does not reduce the risk of myocardial infarction or cardiovascular death. The reduction in strokes over 10 years (number needed to treat = 444) must be balanced against an increase in serious gastrointestinal bleeds (number needed to treat to harm = 553). No change in all cause mortality was seen in this large, long study.

Level of evidence 1b (see Individual randomised controlled trials (with narrow confidence interval)

Ridker PM, Cook NR, Lee IM, et al. A randomized trial of low-dose aspirin in the primary prevention of cardiovascular disease in women. N Engl J Med 2005;352: 1293-304.

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  • * Patient-Oriented Evidence that Matters. See editorial (BMJ 2002;325: 983)

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