Comparing the Safety and Efficacy of Apixaban and Rivaroxaban

Inclusion Criteria: In order to be eligible to participate in this study, an individual must meet all of the following criteria: 1. Male or female Veteran 2. Diagnosis of atrial fibrillation (AF) or atrial flutter (AFL) and currently taking either apixaban or rivaroxaban. 3. CHA2DS2-Vasc of 3 or more 4. Ability to take oral medication and self-reported willingness to adhere to the prespecified apixaban or rivaroxaban regimen Exclusion Criteria: An individual who meets any of the following criteria will be excluded from participation in this study; notably use of antiplatelet agents or prior OAC use will not be an exclusion criterion: 1. Current use of oral or injectable anticoagulation, without ability to switch to the assigned study medication 2. Another indication for anticoagulation, such as pulmonary embolism 3. Contraindication to oral anticoagulation 4. Known bleeding diathesis 5. Documented current pregnancy or lactation in the EHR or self-reported current pregnancy or lactation 6. Known allergic reactions or intolerance to apixaban or rivaroxaban 7. Most recent estimated glomerular filtration rate (eGFR) is \< 30 mL/minute/1.73m2. An eGFR result must have been obtained within 18 months before randomization. 8. Known mechanical heart valve 9. Known moderate-severe mitral stenosis 10. Known history of left atrial occlusion, excision, or ligation 11. Current or planned use of systemic ritonavir, itraconazole, or ketoconazole (topical use of ketoconazole only is allowed) 12. Cardiac or thoracic surgery in the past 3 months