Atrial fibrillation affects about 1.2% of the population in the United Kingdom and accounts for about a sixth of all strokes.1 2 Its prevalence increases steeply with age, from 0.5% of those aged 50-59 years to 10% of those over 80.1 Strokes are more disabling in patients with atrial fibrillation and have higher 30 day mortality than those of arterial origin.3 4 Because of the ageing population, the burden of stroke caused by atrial fibrillation is expected to rise sharply over the next few decades unless more effective thromboprophylaxis can be given to the population at risk.3

When compared with placebo or no antithrombotic treatment, warfarin reduces the risk of stroke by about two thirds in patients with atrial fibrillation,5 but this drug is underused because it is inconvenient and causes bleeding. Antiplatelet treatment with aspirin is much less effective than warfarin—it reduces the risk of stroke by about a fifth compared with placebo.5 Adding clopidogrel to aspirin improves the effectiveness of antiplatelet treatment to prevent stroke but the combination remains significantly less effective than warfarin.6 7 Current guidelines recommend warfarin for patients with atrial fibrillation at high risk of stroke (previous stroke or embolism or more than one of the following risk factors: age ≥75 years, hypertension, diabetes, or congestive cardiac failure), either …