Aspirin for the Prevention of Recurrent Venous Thromboembolism

Inclusion Criteria: * first episode of symptomatic, objectively confirmed idiopathic proximal deep vein thrombosis and/or pulmonary embolism; * initial treatment with unfractionated heparin or low-molecular-weight heparin (or effective alternative) followed by a vitamin K antagonist (target INR 2.0-3.0). All patients will receive 6 or 12 months of oral anticoagulant treatment. Patients initially treated with thrombolytic therapy who received warfarin therapy are eligible for inclusion. Exclusion Criteria: * permanent risk factors for venous thromboembolism: patients known to have antiphospholipid antibodies or lupus anticoagulant (based on local laboratory criteria) or to have homozygous factor V Leiden or homozygous prothrombin G21210A or heterozygous factor V Leiden plus heterozygous prothrombin G21210A or antithrombin III deficiency; patients with active malignancy * temporary risk factors for venous thromboembolism * any recurrence of venous thromboembolism or bleeding episode during the established 6-month period of oral anticoagulant treatment * allergy or intolerance of aspirin * clear indication for aspirin or other anti-platelet therapy (e.g. clopidogrel, ticlopidine) * clear indication for long-term anticoagulant therapy (e.g. recurrent idiopathic venous thromboembolism, prosthetic heart valve) * treatment with non-selective COX-1/2 non-steroidal anti-inflammatory drugs * life expectancy less than 6 months * active bleeding or at high risk of bleeding (gastrointe…