Vasopressin is a man-made form of a hormone called "anti-diuretic hormone" that is normally secreted by the pituitary gland. Vasopressin acts on the kidneys and blood vessels. Vasopressin helps prevent loss of water from the body by reducing urine output and helping the kidneys reabsorb water into the body. Vasopressin also raises blood pressure by narrowing blood vessels. Vasopressin is used to treat diabetes insipidus, which is caused by a lack of this naturally occurring pituitary hormone in the body. Vasopressin is also used to treat or prevent certain conditions of the stomach after surgery or during abdominal x-rays. Vasopressin may also be used for purposes not listed in this medication guide.