List of products in the National Drug Code with proprietary name . Metolazone is a "water pill" (diuretic) that increases the amount of urine you make, which causes your body to get rid of excess water. This drug is used to treat high blood pressure. Lowering high blood pressure helps prevent strokes, heart attacks, and kidney problems. This medication also reduces swelling/fluid retention (edema) which can result from conditions such as congestive heart failure or kidney disease. This can help to improve symptoms such as trouble breathing.