Diuretics

The three most common types of diuretics are:

* thiazides (eg bendroflumethiazide)
* loop diuretics (eg furosemide)
* potassium-sparing diuretics (eg amiloride)

Thiazide diuretics

Thiazide diuretics are classed as "moderately potent" diuretics. They are often used to treat high blood pressure, especially in older people. See Common diuretics.

A low dose of a thiazide diuretic (for example 2.5mg of bendroflumethiazide) is often the first thing your doctor will prescribe if you have a slightly high blood pressure. This can be taken on its own or in combination with another blood-pressure medicine such as an ACE inhibitor or beta-blocker. For more information please see the separate Bupa health factsheets, ACE inhibitors and Beta-blockers.

Loop diuretics

These medicines are named after the loop of Henlé, one of the tubes inside the kidney. Loop diuretics are powerful diuretics that are usually used for treating heart failure.

The main effect of loop diuretics is to make the kidneys re-absorb less salt and water, and so produce more urine. Removing the excess water gives the heart less pumping work to do and reduces any breathlessness or ankle swelling.

Potassium-sparing diuretics

These medicines are named after their action, which helps to prevent the removal of potassium from the body by diuretics. Potassium has many important roles in your body, including keeping your heart healthy and your blood pressure at a normal level.

Potassium-sparing diuretics are weak when used alone but are often given in combination with a thiazide diuretic to prevent low potassium levels in the body (hypokalaemia).

They work on the salt balance (the balance of sodium and potassium chloride) within the kidneys. They do this by increasing the amount of sodium that is lost from the body in the urine and reducing the amount of potassium lost in the urine. This helps to maintain the potassium levels in the body.

How do diuretics work?

Thiazides, loop diuretics and potassium-sparing diuretics all work on the kidneys.

The kidneys have a network of tubes that make urine by filtering the blood in two stages. In the first stage, the water, salt and the waste products such as urea are filtered out from the blood, leaving behind the red and white blood cells. But lots of nutrients and other essential substances also leave the blood at this stage.

In the second stage there is a backwards filter, where the kidneys re-absorb the nutrients and essential substances back into the blood. This leaves the waste products, plus some salt and water, in the kidneys. This waste leaves the kidneys as urine.

Heart failure makes the kidneys re-absorb more water and salt into the blood, and so produce less urine. This is the body's way of trying to compensate for the reduced pumping power of the heart, but it actually makes matters worse. There is a bigger volume of blood for the heart to pump, and so more work for it to do. Also the excess water in the blood tends to leak out into the lungs, which can lead to breathlessness. Heart failure can also make your ankles and feet swell up.

Diuretics reduce the amount of water and salt that is re-absorbed by your kidneys. So, more water and salt passes out with the waste products into your urine. This means that you make more urine. As a result of this loss of water from the blood, the overall volume of blood is reduced. This gives your heart less work to do and helps to reduce the pressure of blood in your arteries, which is what high blood pressure actually is.
How to take a diuretic

Most diuretics come in the form of tablets (known as "water tablets"). It's best to take these in the morning or early afternoon so that the extra urine your kidneys produce doesn't interfere with your sleep. Diuretics also come as injections but most people only take water tablets.

Because loop diuretics can cause blood potassium levels to fall, your doctor may advise you to take a supplement of potassium. You can also help keep your potassium levels up by eating foods with a high potassium content. Bananas are a good source of potassium.

Side-effects

You may not notice any side-effects with a low dose of a thiazide diuretic. At higher doses, you may get an upset stomach or feel faint or dizzy when you stand up. This can be caused by low blood pressure.

The first time you take a loop diuretic you may need to pass a very large volume of urine and take regular trips to the toilet. Your doctor may advise you not to take this medicine in the evening, as it will increase the number of times you have to get up in the night to go to the toilet.

Loop diuretics can also make you feel dizzy, especially during the first few days of treatment. Other, less common, side-effects can include:

* feeling sick
* digestive problems
* gout
* muscle pain
* skin rash

Loop diuretics can also upset the balance of minerals in the body, especially potassium. See How to take a diuretic.