Tolvaptan is a vasopressin receptor antagonist. Vasopressin (a.k.a. ADH or antidiuretic hormone) helps to regulate water retention by absorbing water in the collecting ducts of the nephron.

According to Palmer (2000), SIADH is an expansion of extracellular fluid volume resulting from the superfluous release of antidiuretic hormone ... can enhance the action of ADH (Albanese et ...