Anticoagulant therapy remains the mainstay of medical therapy for deep venous thrombosis (DVT) because it is noninvasive, it treats most patients (approximately 90%) with no immediate demonstrable physical sequelae of DVT, it has a low risk of complications, and its outcome data demonstrate an improvement in morbidity and mortality. Meta-analyses of randomized trials of unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) have shown that they are similar, with a 4% risk of recurrent DVT, a 2% risk of pulmonary embolism (PE), and a 3% risk of major bleeding.
Initial Anticoagulation Therapy
First-line therapy for non-high risk venous thromboembolism (VTE) or pulmonary embolism (PE) consists of direct oral anticoagulants (dabigatran, rivaroxaban, apixaban, or edoxaban) over VKAs. [3, 4] Vitamin K antagonists (VKAs) are also recommended over low-molecular-weight heparin (LMWH), unless VTE is associated with malignancy, in which case LMWH is preferred over VKAs or any direct oral anticoagulants. 
Note that it is not recommended that inferior vena cava filters be placed in patients with acute VTE who are on anticoagulant therapy. 
Initial anticoagulation therapy to treat DVT traditionally involves continuous intravenous (IV) heparin until adequate systemic anticoagulation is achieved. Rapid anticoagulation is essential within the first 24 hours of diagnosis, reducing the incidence of recurrent venous thrombosis during the first 3 months from 25% to 5%.[5, 6]
Continuous IV heparin for therapy initiation has been increasingly replaced by single or twice-daily subcutaneous (SC) injections of LMWH. LMWH antithrombotic effects correlate with body weight and permit fixed dosing without laboratory monitoring; LMWH also allows for outpatient treatment of uncomplicated DVT. [7, 8,9] However, IV heparin remains the treatment of choice for those with end-stage renal failure.
Guidelines recommend short-term anticoagulation with LMWH SC, unfractionated heparin (UFH) IV, fixed-dose UFH SC, or fondaparinux SC.  Initial treatment with LMWH, UFH, or fondaparinux should continue for at least 5 days and until the international normalized ratio (INR) is 2 or higher for at least 24 hours. A vitamin K antagonist (VKA) such as warfarin should be initiated together with LMWH, UFH, or fondaparinux on the first treatment day. 
Patients with recurrent VTE while on treatment with a non-LMWH anticoagulant should be switched to LMWH therapy.  Those who suffer recurrent VTE while on LMWH therapy should receive an increased dose of LMWH. 
Factor Xa and direct thrombin inhibitors
Rivaroxaban (Xarelto) is an oral factor Xa inhibitor approved by the FDA in November 2012 for treatment of DVT or PE and for reduction of the risk of recurrent DVT and PE after initial treatment. [11, 12] Approval for this indication was based on studies totaling 9478 patients with DVT or PE. Participants were randomly assigned to receive rivaroxaban, a combination of enoxaparin and a VKA (eg, warfarin), or a placebo. Study endpoints were designed to measure the number of patients who experienced recurrent symptoms of DVT, PE, or death after receiving treatment.
Data from a pooled analysis of the EINSTEIN-DV  and EINSTEIN-PE  trials suggested that use of rivaroxaban is as effective in preventing venous thromboembolism (VTE) recurrence as administering enoxaparin followed by a VKA, and it may be associated with less bleeding; in addition, the data suggested that there are no grounds for avoiding rivaroxaban use in high-risk groups (eg, fragile patients, cancer patients, and patients with a large clot).
Approximately 2.1% of patients treated with rivaroxaban experienced recurrent DVT or PE, compared with 1.8-3% treated with the enoxaparin and VKA combination. [11,12] In addition, results from extended treatment demonstrated a reduced risk of recurrent DVT and PE. Approximately 1.3% in the rivaroxaban group experienced recurrent DVT or PE, compared with 7.1% in the placebo group. [13, 14]
In arch 2014, the FDA approved apixaban (Eliquis) for the additional indication of prophylaxis of DVT and PE in adults who have undergone hip- or knee-replacement surgery. Support for this new indication was a result of the ADVANCE 1, 2, and 3 clinical trials that enrolled nearly 12,000 patients. [15, 16, 17] Apixaban was originally approved by the FDA in December 2012 for the prevention of stroke and systemic embolism in patients with nonvalvular atrial fibrillation (NVAF).
In August 2014, apixaban was approved for treatment of DVT and PE. The approval for treatment of PE and prevention of recurrence was based on the outcome of the AMPLIFY (Apixaban for the Initial Management of Pulmonary Embolism and Deep-Vein Thrombosis as First-Line Therapy) and AMPLIFY-EXT studies, in which apixaban therapy was compared with enoxaparin and warfarin treatment. The AMPLIFY study showed that, in comparison with the standard anticoagulant regimen, apixaban therapy resulted in a 16% reduction in the risk of a composite endpoint that included recurrent symptomatic VTE or VTE-associated death. [18, 19]
Data from the AMPLIFY-EXT trial showed that extended anticoagulation (12 months) with apixaban shortened hospital stays, reduced symptomatic recurrent venous thromboembolism or all-cause death without an associated increase in major episodes of hemorrhage when compared with placebo. 
Dabigatran (Pradaxa) inhibits free and clot-bound thrombin and thrombin-induced platelet aggregation. It was FDA approved in 2010 to reduce the risk of stroke in patients with NVAF. In April 2014, the FDA approved it for the treatment of DVT and PE in patients who have been treated with a parenteral anticoagulant for 5-10 days. In addition, it was approved to reduce the risk of DVT and PE recurrence in patients who have been previously treated. Approval was based on results from four global phase III trials that showed dabigatran was noninferior to warfarin and had a lower risk of major or clinically relevant bleeding compared with warfarin. [21, 22, 23] There have been reports of severe and fatal bleeding in users of the drug.
The RE-COVER and RE-COVER II trials included patients with DVT and PE who were treated with parenteral anticoagulant therapy for 5-10 days. Results showed dabigatran was noninferior to warfarin in reducing DVT and PE after a median of 174 days of treatment with a lower risk of bleeding compared with warfarin. [21, 22]
The RE-SONATE trial and RE-MEDY trials included 2856 patients with acute DVT and PE who had completed at least 3 months of anticoagulant therapy. Results from this trial showed dabigatran was noninferior to warfarin in the extended treatment of VTE and carried a lower risk of major or clinically relevant bleeding than warfarin.
Edoxaban (Savaysa) was approved by the FDA in January 2015 for treatment of DVT and PE in patients who have been initially treated with a parenteral anticoagulant for 5-10 days. Approval was based on the Hokusai-VTE study that included 4,921 patients with DVT and 3,319 patients with PE.  Among patients with PE, 938 had right ventricular dysfunction, as assessed by measurement of N-terminal pro-brain natriuretic peptide (NT-proBNP) levels. The rate of recurrent VTE in this subgroup was 3.3% in the edoxaban group and 6.2% in the warfarin group. Edoxaban was noninferior to high-quality standard warfarin therapy and caused significantly less bleeding in a broad spectrum of patients with VTE, including those with severe PE. 
Betrixaban (Bevyxxa), a FXa inhibitor, was approved by the FDA in June 2017.  It is indicated for the prophylaxis of VTE in adults hospitalized for acute medical illness who are at risk for thromboembolic complications owing to moderate or severe restricted mobility and other risk factors that may cause VTE. 
Approval of betrixaban was based on data from the phase 3 APEX studies. [26, 27]These randomized, double-blind, multinational clinical trials compared extended-duration betrixaban (35-42 days) to short-duration enoxaparin (6-14 days) for VTE in 7,513 acutely medically ill hospitalized patients with VTE risk factors. [25, 26, 27]Patients in the betrixaban group received an initial dose of 160 mg orally on day 1, followed by 80 mg once daily for 35-42 days, and received a placebo injection once daily for 6-14 days. Patients in the enoxaparin group received 40 mg subcutaneously once daily for 6-14 days and took an oral placebo once daily for 35-42 days. [25, 26, 27]
Efficacy was measured in 7,441 patients using a composite outcome score composed of the occurrence of asymptomatic or symptomatic proximal DVT, nonfatal PE, stroke, or VTE-related death. [25, 26, 27] Those who received betrixaban showed significant decreases in VTE events (4.4%) compared with patients in the enoxaparin group (6%).