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Diuretics, sometimes known as ‘water pills’, are drugs which draw excess fluid from the tissues of the body and convert it into urine. They are used for the swelling and bloating of premenstrual syndrome, for treating high blood pressure and, in older people, for heart failure caused by weakening of the heart’s pumping mechanism. They are also prescribed for nephrotic syndrome, liver damage, glaucoma and Méniêre’s disease.

The drugs work by disrupting the normal action of the kidneys. The kidneys usually remove water, minerals and waste products from the bloodstream. Most of the water and minerals are returned to the bloodstream after the waste products have been expelled in the urine. Diuretics reduce the amounts of sodium and water reabsorbed into the bloodstream, which increases the amount of urine, draws excess fluid from tissues and reduces the water content of the blood. As the diuretics act to expel urine from the body, the tissues become less water-logged and the action of the heart improves because it has to pump a smaller volume of blood around the body. This, in turn, acts to reduce blood pressure. Different diuretic drugs work in slightly different ways, but all make you go to the bathroom more often. If you suffer swelling before your period, for example, you will feel less bloated. Sufferers from heart failure will feel less breathless as the diuretic removes fluid that has gathered in the lungs and places less strain on the heart.

Mild diuretics and herbal tablets that have a diuretic action are available over-the-counter for the treatment of premenstrual syndrome and minor cases of fluid retention. In more serious conditions they should be prescribed by a doctor. There are various types, all of which have a slightly different action.

THIAZIDES
These are the type prescribed most often and they work by blocking the reabsorption of sodium, potassium and water. Taking them, however, can lead to a shortage of potassium in the body and for this reason they may be given with a potassium supplement or in conjunction with another type of diuretic known as a potassium-sparing diuretic.

LOOP DIURETICS
These are powerful diuretics which also block the reabsorption of sodium, potassium and water. They have an extremely rapid action and for this reason they are sometimes used in emergencies, for example to relieve fluid on the lungs. They are especially useful to people with impaired kidney function who do not react well to thiazide diuretics. Like thiazides they can cause a shortage of potassium, which can be counteracted by taking either a potassium supplement or a potassium-sparing diuretic.

POTASSIUM-SPARING DIURETICS
As the name suggests, these diuretics block the reabsorpton of sodium and water without affecting the body’s potassium balance. They are mild diuretics and may be prescribed together with a thiazide or loop diuretic to prevent the body losing excess potassium.

OSMOTIC DIURETICS
These are not prescribed very often. They work by blocking the reabsorption of sodium and water and are used to maintain the flow of urine through the kidneys after surgery or injury and to lower pressure within fluid-filled cavities

ACETAZOLAMIDE
This is a mild diuretic that is mainly used to treat congestive heart failure, fluid retention, pre-eclamptic toxaemia (PET) during pregnancy, premenstrual tension and epilepsy. It’s also used to treat some types of glaucoma when locally acting drugs have failed to treat the problem successfully.

CARBONIC ANHYDRASE INHIBITORS
These work by blocking the effect of carbonic anhydrase, an enzyme that affects the level of bicarbonate ions in the blood. However, they are only effective for a short period of time.