The goal of this clinical trial, called a pilot study or a feasibility study, is to test the study plan and to find out whether enough participants will join a larger study and accept the study procedures. This type of study includes a small number of participants so it is not expected to prove how safe the treatment is or how well the treatment works.

The main question it hopes to answer is:

1.What is the average number of patients that are recruited per month during the 12 month study period?

To test the study plan, adults being treated for a superficial vein thrombosis (SVT), which is a blood clot in the superficial veins of the leg, will be given a type of blood thinner called rivaroxaban. Half of the participants in this study will be given the standard (low-dose) rivaroxaban for 45 days, then 45 days of placebo (a substance that looks like the study medication but does not have any active or medicinal ingredients). The other half of participants will be given full-dose rivaroxaban for a total of 90 days. The placebo in this study is not intended to have any effect on the participants blood clot. A placebo is used to make the results of the study more reliable.

The standard or usual treatment for a superficial vein thrombosis (SVT) is to treat with a low-dose (often called a prophylactic dose) of blood thinner for 45 days. Rivaroxaban is a type of oral blood thinner that is used in the treatment and prevention of blood clots. Currently, rivaroxaban 10mg daily for 45 days is the most commonly used treatment in patients with lower extremity SVT.

Recent studies of participants with SVT suggest that treatment with full-dose (sometimes called therapeutic dose) blood thinners could be promising in preventing additional complications from SVT such as:

• A new blood clot in the deep veins of the legs or arms
• A new pulmonary embolism (a blood clot in the lungs)
• A recurrence or extension of the existing SVT (the existing clot comes back after treatment is stopped or gets bigger)

The Investigators are studying whether a full-dose of rivaroxaban for 90 days could prevent or improve additional complications from SVT.

Inclusion Criteria:

1. Adult patients age ≥ 18 years old.
2. Objectively confirmed diagnosis within 14 days of an acute symptomatic SVT of the lower extremities by standardized CUS, where SVT is defined as incompressibility of a venous segment located along the course of a known superficial vein.
3. Anticoagulation for SVT is warranted per clinicians.
4. Able and willing to provide written informed consent.

Exclusion Criteria:

1. Other indication(s) for therapeutic or prophylactic dose anticoagulation (e.g. atrial fibrillation, mechanical valve, etc.).
2. History of PE or DVT within 6 months (180 days) of screening.
3. >5 days of any anticoagulants for the index SVT.
4. Requires use of aspirin >100mg daily or other antiplatelet agents.
5. Patients receiving concomitant systemic treatment with strong inhibitors of both CYP 3A4 and P-gp (e.g. cobicistat, ketoconazole, itraconazole, posaconazole, ritanovir, etc.).
6. Active bleeding or history of CRNMB or major bleeding (as defined by the ISTH) within 30 days of screening.
7. History of severe head trauma or ophthalmic, spinal, cerebral surgery within 90 days of screening.
8. Have acute endocarditis.
9. Thrombocytopenia (platelet count <50,000/uL), acute hepatitis, chronic active hepatitis, cirrhosis with severe hepatic impairment defined by a Childs-Pugh class B or C.
10. Creatinine clearance <30 ml/min.
11. Known contraindication to treatment with rivaroxaban.
12. Are participating in another interventional trial that would compromise the results or integrity of this trial as determined by the investigator.
13. Pregnant or breast feeding.
14. Known hereditary or acquired severe hemorrhagic disease.
15. Life expectancy <3 months.
16. Unstable medical or psychological condition that would interfere with trial participation at the discretion of the site investigator.