Diuretics (water pills) can cause the levels of potassium in the blood to decrease so this is why a person taking diuretics may need extra potassium. However, there are some diuretics that do not cause potassium loss.
Diuretics lower blood pressure by helping your body eliminate sodium and water, which reduces blood volume and decreases pressure on your artery walls. When your body excretes excessive amounts of water, it also loses potassium.
When a person has low levels of potassium they have what is called hypokalemia. Potassium can be replenished through diet and supplements as well.
Diuretics act on the renal tubles (target organ), to increase urinary output. Increased urine output means that substances that are lost in the urine are also lost in increased amounts.
Some diuretics (not all), are "potassium sparring". Some make you lose potassium and others do not. If the particular diuretic being taken is one that makes you lose potassium, then the potassium must be replaced. Potassium has many important functions, one of which is helping to propigate neurological (nervous) impulses in muscle tissue (including the heart). Not replacing the lost potassium can have an impact on muscular contractions.
Your physician is best suited to decide which, if any diuretic therapy and potassium supplementation is right for you.
A diuretic is basically a drug that increases the rate of urination. As a person urinates more there is a loss of bodily salts with urine. Different compounds which are expelled with urine have a function to play in the transmission of signals from the brain, action of muscles and many other functions in the body. The decrease of any compound could adversely affect the processes in the body, like breathing, beating of the heart and others.
This is the reason why a person taking a diuretic has to take more potassium. So that processes which require potassium are not affected.
Though, this is not the case always, there are many different types of diuretics and each has a different effect on the rate of substances expelled from the body, for examples Aldosterone antagonists and Epithelial sodium channel blockers decrease the secretion of potassium with urine. If these are used, it may not be necessary to take potassium.