Lasix is approved to treat high blood pressure and fluid retention by increasing the amount of salt and water the kidneys remove from the blood. As a result, Lasix causes a decrease in blood volume, which can lower blood pressure and also help with water retention. Although Lasix cannot cure these conditions, it can help to decrease the risk of developing problems that can occur with long-term high blood pressure, such as heart disease or congestive heart failure. Some off-label Lasix uses include treating hypercalcemia and water retention due to PMS.