Rivaroxaban Effects in Subjects Who Have Undergone Gastric Bypass Surgery

The investigators are attempting to compare the anticoagulant (blood thinning) effects of a recently FDA approved medication, Rivaroxaban, in people who have undergone gastric bypass surgery to people who have not had this surgery. It is thought that gastric bypass may alter the absorption of this medication, but this has not been previously studied. The investigators will study the anticoagulant effect of rivaroxaban in healthy volunteers who are in one of four groups 1. Non obese people who have not had a gastric bypass. 2. Obese people who have not had a gastric bypass 3. People who have had a gastric bypass 4. People who are planning to undergo gastric bypass surgery in the near future who are willing to be studied before and after the bypass. The study will involve taking a single low dose of rivaroxaban and multiple blood samples will be taken over the next 24 hours and the effect of rivaroxaban on blood clotting will be measured using the prothrombin time and an anti-factor Xa assay. The effects of rivaroxaban will be compared between the different groups.