Tolvaptan 15 mg is a medication that belongs to the class of drugs known as selective vasopressin V2 receptor antagonists. It is primarily used to treat a condition called hyponatremia, which is characterized by low sodium levels in the blood. Tolvaptan is a medication used to treat hyponatremia (low sodium levels in the blood) and certain types of fluid retention. However, as with any medication, it can have potential side effects and warnings to be aware of.