To maintain sodium-potassium balance. Furosemide (Lasix) is a loop diuretic and spiranolactone is a potassium-sparing diuretic.Why would she be on two diuretics?
In a nutshell, at high dosages Furosemide can cause low blood potassium levels (hypokalemia), which can cause unwanted side effects such as weakness, severe muscle cramps, cardiac arrest and death.
And at high dosages spiranolactone can cause high blood potassium levels (hyperkalemia),which can cause unwanted side effects such as nausea, vomiting, severe abdominal pain, cardiac arrest and death.
However, when taken together, they can keep your potassium levels from getting way out of whack and killing you.
And I'm telling you guys the same thing I used to tell my competitive athletes when I was a personal trainer/nutritionist. I highly recommend that you do not take these drugs unless prescribed by a your doctor, but if are going to take them anyways, the correct ratio is 100:40. 100mg of spiranolactone with every 40mg of furosemide.