Tolvaptan is one of new class of medications approved for treatment of low blood sodium (hyponatremia). It works by interfering with the effect of the antidiuretic hormone, vasopressin, thereby increasing urine output and decreasing body water. However, the magnitude of these effects vary from person to person. Thus, the efficacy of tolvaptan is less reliable than an infusion of a concentrated salt solution for emergency, short term treatment of hyponatremia.