Introduction: Vitamin B12 (Cyanocobalamin) deficiency is a common vitamin deficiency that can present with a range of clinical manifestations, including pancytopenia. This case illustrates the presentation of a common vitamin deficiency as pancytopenia that can cause higher morbidity in otherwise healthy patients. Case Description: A 61-year-old female presented to the emergency department (ED) with generalized weakness. She recently moved from Puerto Rico and has medical history of chronic anemia, required blood transfusions in the past. She denied any melena, easy bruising or neurological symptoms. Initial work up in the ED showed significant pancytopenia with WBC count 0.5, Hemoglobin 4.3, and platelet count 37 K. MCV was elevated at 123, peripheral blood smear showed hyper-segmented neutrophils. She received 2 units of packed RBCs and was admitted to the hospital. Hematology was consulted for further work up of significant pancytopenia. Bone marrow biopsy was pursued, which showed markedly hypercellular bone marrow consistent with megaloblastic anemia in the setting of vitamin B12 deficiency. No obvious myelodysplasia or malignancy noted. B12 level was found to be/ml (normal levels-211-911 pg/ml), folate level was normal. Patient had no symptoms of malbasorption, and did not follow a vegetarian diet. Work up for pernicious anemia, including both parietal cell and intrinsic factor antibodies was negative. Patient was treated with a course of intramascular injections of vitamin B12. Follow up with Hematology as outpatient showed significant improvement in the cell count and clinical symptoms. Conclusion: Although vitamin B12 is a common deficiency, a clinician must be aware of the rare manifestations of this condition to adequately manage the patients. Including it the differential diagnosis, work up of pancytopenia is essential for the early management.