How Does Furosemide Work? Furosemide ( Lasix ® ) is a prescription medication approved for the treatment of high blood pressure. It can also be used to treat water retention caused by kidney disease or kidney failure, congestive heart failure , or cirrhosis. It may take 2 weeks or so before the optimal dose of lasix is found for your dog. So begin with the low dose for a few days and if you do not notice any improvement please ask your vet when it will be time to increase the dose. for so long. He saved my life. Was diagnosed by him with water retention, given lasix alone for 10 days and lost 10 pounds. Then was prescribed a potassium to take along. Part of this first diagnosis also wass. How to use Lasix. Read the Patient Information Leaflet if available from your pharmacist before you start taking furosemide and each time you get a refill. The onset of increased urine production (diuresis) after taking of Lasix orally is within 1 hour. The peak diuretic effect occurs within the first or second hour which then reduces over the next 6 to 8 hours. The half life of lasix is two hours, as to how long will a patient continue to urinate. I do not have that answer. Furosemide (Lasix), a diuretic, works quickly to relieve edema, or fluid retention. About an hour after you take it, you'll find that you need to urinate. How long does it take for. How long does it take for ankle swelling to reduce after taking Lasix? Asked 18 Apr 2014 by 11372ny Updated 18 April 2018 Topics lasix, fluid retention, swelling. Answer this question. Responses (2) KA. kaismama 19 Apr 2014. Lasix starts working fairly rapidly and you'll find you're going to the bathroom allot. The edema will decrease in a few hours and after a day.