Lasix (furosemide) is a loop diuretic (water pill) that prevents your body from absorbing too much salt. This allows the salt to instead be passed in your urine. Lasix is used to treat fluid retention (edema) in people with congestive heart failure, liver disease, or a kidney disorder such as nephrotic syndrome. Lasix is also used to treat high blood pressure (hypertension). You should not use Lasix if you are unable to urinate.