Given the limited long-term effectiveness of traditional weight loss methods, bariatric surgery is increasingly becoming the preferred option for sustained weight loss. With the ascendancy of the laparoscopic approach, the two most common procedures are the Roux-en-Y gastric bypass (RYGB) and the vertical sleeve gastrectomy (VSG). Because bariatric surgery decreases nutrient intake through restriction, malabsorption, or both, and given that obese patients are often malnourished even before surgery, postoperative micronutrient deficiency, particularly of iron, can be a serious complication and difficult to treat. Iron deficiency anemia has been reported to be as high as 49% in the post-bariatric surgical patient. The current standard for correcting iron deficiency anemia in the post-operative bariatric surgical patient is oral iron supplements. However, oral iron therapy is known for its caustic effects on the gastric mucosa causing gastric irritation, nausea, epigastric discomfort and constipation. These debilitating symptoms lead to poor adherence and lower long and short-term efficacy. Furthermore, iron absorption from oral iron supplements when taken with food in patients with low iron stores ranges from 2 to 13% and without food 5 to 28%. An alternative and more effective method of iron replenishment is the use of intravenous iron. A litany of published trials, without contradiction, show marked superiority of intravenous iron in improving hemoglobin concentrations and iron parameters when compared to historical controls. Nonetheless, the current recommendations of the American Society of Metabolic and Bariatric Surgery nutritional guidelines, state that oral iron supplementation for IDA is the recommended first line of treatment. Studies are lacking that compare the efficacy of oral versus intravenous (IV) iron therapy for the treatment of IDA in the post-bariatric surgical patient. The aim of our study is to compare two accepted treatments for iron deficiency anemia (oral ferrous sulfate and intravenous ferumoxytol) for efficacy and speed of response in the treatment of IDA in the post-operative bariatric surgical patient. In this study, 104 bariatric surgical post-operative patients will be randomly assigned 52 each to oral or 52 to a single dose IV iron treatment using double-blind procedures. The primary outcome will be determined at 6 weeks of treatment with a follow-up at 12 months after treatment. Non-responders at 6 weeks after treatment may, if they qualify (based on inclusion/exclusion criteria), have an open-label IV iron treatment and will be followed with the same evaluations used after the first IV iron treatments.