For vitamin B12 to be effectively absorbed from the intestine it needs to be bound to a substance produced in the stomach called “intrinsic factor.” After gastric bypass the B12 in the patients’ diet is less effectively bound to intrinsic factor and therefore may not be picked up as well. Vitamin B12 deficiencies are not common, but can occur slowly, several years after surgery. Profound deficiency – < 250pg/dl – is treated by either injection or application of a spray to the nasal lining (NASCOBAL) for several months. Patients in the low normal range 250-400 pg/dl should take a vitamin formulation specifically for B12 deficiency. After the deficiency has been corrected the patient should take a supplemental B complex vitamin formulation from then on.