Depending on the amount of fluid a cat is suspectible to collecting, lasix alone may not be enough. A cat may also need spironolactone not only as a diuretic but also to help the body preserve potassium. If the CBC/chem panel blood test shows a low level of potassium, around 4 or less, then a supplement might be needed. Normal range is 3.5-5 for cats. My cat's blood potassium was 4.2 but the cardiologist-in order to warrant off any possible kidney issues since my cat's on lasix and spironolactone-suggested I give a supplement. I have been giving her half of a human potassium supplement (divided over three meal times) and her potassium level is now 4.5. If you are trying to decide if your cat should have both, I would say both are beneficial. Good luck!
[Non-text portions of this message have been removed]