Diuretics work by preventing the reabsorption of salt and water from the urine while it is being formed in the kidneys. Consequently the volume of urine increases, and the amount of salt and water in the body is correspondingly reduced. They thus produce similar effects to going on a low salt diet.
There are three basic types of diuretics.
The most widely used for the treatment of high blood pressure are the thiazides. They have been used in all the major trials, which have shown that treating high blood pressure prevents strokes and heart attacks, and continue to be drugs of first choice, particularly in the elderly. Thiazide diuretics tend to cause loss of potassium in the urine, which may require increasing potassium intake. This can be done either by eating potassium-rich food (bananas are the classic example) or by adding a potassium pill. Contrary to popular legend, they do not cause kidney damage.
The second type is the potassium-retaining diuretic. The most powerful of these is aldactone, which works by antagonizing the effects of a salt-retaining hormone, aldosterone. Aldactone makes the kidneys excrete more salt and also retain potassium. When given in large doses, it can cause painful swelling of the breasts. It is used on its own or in combination with thiazide diuretics. The other potassium-retaining diuretics are not as strong and are mainly used in combination with thiazides to counteract their potassium-losing effect. Dyazide is an example of such a combination medication.
The third type is the 'loop' diuretic. Urine is made in the kidneys by being filtered and concentrated through millions of tiny tubules, one part of which is called the loop of Henle, which is where these diuretics act. Loop diuretics are the most powerful of the three types and are usually used only when kidney function is impaired. A commonly used example is lasix.
