Six RCTs (n=9,853) were included. All studies generated randomised treatment allocation sequences. Five trials used a fixed aspirin dose. Four trials had a blinded outcome assessment. Five trials reported more than 99% follow-up. Four trials were completely or partially funded by pharmaceutical companies. Duration of follow-up ranged from 13 to 50 months. There was no evidence of publication bias.
Aspirin therapy resulted in significant reductions for: all cardiovascular events combined (OR 0.79, 95% CI 0.72 to 0.88, NNT=30), risk of nonfatal myocardial infarction (OR 0.74, 95% CI 0.60 to 0.91, NNT=83), risk of stroke (OR 0.75, 95% CI 0.65 to 0.87, NNT=40) and risk of all-cause mortality (OR 0.87, 95% CI, 0.76 to 0.98, NNT=71). Patients treated with aspirin were significantly more likely to experience severe bleeding (OR 2.2, 95% CI 1.4 to 3.4, NNH=111).
Neither aspirin dose nor length of follow-up affected any clinical outcome. Patients with ischaemic heart disease showed significant reductions in risk of major cardiovascular events, all-cause mortality and myocardial infarction (two studies). Studies in which patients were enrolled following cerebrovascular events exhibited a significant reduction in risk of major cardiovascular events (four studies).
There was no evidence of statistical heterogeneity for any comparisons.