Tolvaptan reduces the level of a hormone that regulates the balance of water and salt (sodium) in the body. High levels of this hormone can cause an imbalance that results in low sodium levels and fluid retention. Tolvaptan is used to treat hyponatremia (low levels of sodium in your blood) in people with heart failure, and certain hormonal imbalances. Tolvaptan improves urine flow without causing the body to lose too much sodium as you urinate.