Consented patients undergoing elective total hip and total knee arthroplasty will be randomized to receive either aspirin alone or aspirin and rivaroxaban for prevention of venous thromboembolism.

Aspirin and rivaroxaban prevent venous thromboembolism (VTE) via different mechanisms. Aspirin is significantly cheaper than rivaroxaban. Aspirin in combination with rivaroxaban was shown to be safe and efficacious in a non-inferiority trial (EPCATII) when compared to rivaroxaban alone.

This study will assess if aspirin alone is non-inferior to rivaroxaban and aspirin in the prevention of venous thromboembolism.

Inclusion Criteria:

1. Patients undergoing elective THA/TKA at the participating institutions will be potentially eligible for this study
2. Written informed consent in accordance with federal, local and institutional guidelines

Exclusion Criteria:

1. Previous documented VTE (proximal DVT or any PE)
2. Hip or lower limb fracture in the previous three months, not related to present surgery
3. Metastatic cancer
4. Life expectancy less than 6 months
5. History of major bleeding that in the judgment of the investigator precludes use of anticoagulant prophylaxis
6. History of aspirin allergy, active peptic ulcer disease or gastritis that in the judgment of investigator precludes use of aspirin
7. History of significant hepatic disease or any other condition that in the judgment of the investigator precludes the use of rivaroxaban
8. Creatinine clearance less than 15 ml per minute
9. Pre-operative platelet count less than 100 x 109 /L
10. Need for long-term anticoagulation due to a pre-existing co-morbid condition or due to the development of VTE following surgery but prior to randomization
11. Received anticoagulation post operatively
12. Bilateral THA/TKA or simultaneous hip and knee arthroplasty
13. Major surgical procedure within the previous 3 months
14. Requirement for major surgery post arthroplasty within a 90 day period
15. Chronic daily aspirin use with dose greater than 100 mg a day
16. Women of childbearing potential who are not abstinent or do not use effective contraception or are breast-feeding throughout the study drug period
17. Unwilling or unable to give consent
18. Previous participation in the EPCAT III study
19. Under 18 years of age
20. Concomitant use of drugs that are strong inhibitors of P-gp AND CYP3A4 (e.g., systemic treatment with ketoconazole, itraconazole, or ritonavir) or strong inducers of P-gp AND CYP3A4 (e.g., rifampicin, phenytoin, carbamazepine, phenobarbital, St. John's Wort)
21. Known allergy to food dye

over-encapsulated rivaroxaban/ASA