Inclusion Criteria

1. Age range: ≥ 18 years.
2. Both males and females
3. Confirmed acute calf vein thrombosis confined to either the deep (posterior tibial, anterior tibial, or peroneal) or muscular (gastrocnemius or soleal) veins.
4. Negative serum or urine pregnancy test done (within 2 weeks) prior to randomization, for women of childbearing potential only.Note: A Women of childbearing potential (WOCBP) is defined as any female who has experienced menarche and who has not undergone surgical sterilization (hysterectomy or bilateral oophorectomy) and is not postmenopausal. Menopause is defined as 12 months of amenorrhea in a woman over age 45 years in the absence of other biological or physiological causes.
5. Ability to provide written informed consent.

Exclusion Criteria

1. Any of the following because this study involves an investigational agent whose genotoxic, mutagenic and teratogenic effects on the developing fetus and newborn are unknown:
   • Pregnant women
   • Nursing women
   • Men or women of childbearing potential who are unwilling to employ adequate contraception
2. Acute co-existing proximal DVT (popliteal, femoral, iliac veins or IVC), pulmonary embolism, splanchnic vein thrombosis, cerebral venous sinus thrombosis within the past 3 months for whom anticoagulation therapy is indicated.
3. Age < 18 years.
4. Continuous treatment with therapeutic anticoagulant for more than 72 hours pre-randomization.
5. Contraindication to anticoagulant therapy
6. Significant kidney disease. Creatinine clearance < 25 ml/min using the Cockcroft-Gault equation: GFR = (140-age) * (Wt in kg) * (0.85 if female) / (72 * Cr). (within last four weeks)
7. Significant liver disease (e.g. acute hepatitis, chronic active hepatitis, cirrhosis) or ALT (or AST) > 3 x ULN. (within last four weeks)
8. Platelet count < 50 x109/L.(within last four weeks)
9. Life expectancy < 12 months.
10. Current active bleeding.
11. Concomitant use of strong CYP3A4 inhibitors (e.g., HIV protease inhibitors, systemic ketoconazole) or strong CYP 3A4 inducers like rifampicin.
12. Active cancer defined as any evidence of cancer on cross-sectional imaging or cancer treatments within the past 6 months (chemotherapy, radiation therapy or cancer related surgery).
13. Anticipated need for urgent/emergent surgery or major invasive procedure.
14. Dual antiplatelet therapy (thienopyridine plus aspirin) and/or aspirin greater than 165 mg while on study medication.

Note: Women of child bearing potential must agree to follow instructions for method(s) of contraception for the duration of treatment with study drug(s) plus 33 days after finishing the last dose.