Furosemide will take anything from 1 to 6 hours to reduce swelling. After an hour of taking the drug, your dog will feel like having a pee. This urge to urinate will continue for up to six hours or a bit longer. Furosemide is a diuretic that leads to increased production of urine. And through urinating, the body lets go of the excess amounts of salt and water in the body. Swelling and edema are mostly caused due to kidney or heart failures. Furosemide deals with the swelling and edema by forcing the body to expel the excess salt and water.