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Diuretics

Published by Bupa's health information team, April 2009.

This factsheet is for people who are taking diuretics, or who would like information about them.

Diuretics are medicines that remove water from the body by increasing the amount of urine the kidneys produce. They are often known as 'water tablets' because they remove excess water.

Why would I take diuretics?

Your doctor may prescribe you with a diuretic if you have:

What are the main types of diuretic?

The three most common types of diuretic are:

Thiazide diuretics

Thiazide diuretics are classed as 'moderately potent' diuretics. Your GP may have prescribed these to treat high blood pressure, especially if you're over 60, or 55 if you're of African-Caribbean origin.

A low dose of a thiazide diuretic (for example 2.5mg of bendroflumethiazide daily) is often the first medicine 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 if you have very high blood pressure. For example, your GP may also prescribe angiotensin-converting enzyme (ACE) inhibitors, angiotensin-receptor blockers and calcium-channel blockers.

Higher doses of thiazide diuretics may also be used if you have oedema or heart failure. For oedema, you may initially receive 5 to 10mg of bendroflumethiazide daily. Thiazides recommended for heart failure include bendroflumethiazide, indapamide and metolazone.

Loop diuretics

These medicines are named after the loop of Henlé, one of the tubes inside your kidneys. Loop diuretics are powerful diuretics that are usually used for treating heart failure or too much fluid on your lungs (pulmonary oedema). You may also be given a loop diuretic in addition to your other medicines if you have high blood pressure and other medicines haven't been effective.

The main effect of loop diuretics is to make your kidneys re-absorb less salt and water, and so produce more urine. Removing excess water means that your heart has to pump less and this can reduce symptoms of heart failure, such as breathlessness and ankle swelling.

Loop diuretics recommended for heart failure include bumetanide, furosemide and torasemide.

Potassium-sparing diuretics

These medicines allow water loss from your body but reduce the amount of potassium that is lost at the same time. Potassium has many important roles in your body, including keeping your heart healthy and controlling your blood pressure.

Potassium-sparing diuretics work on the salt balance (the balance of sodium and potassium) 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.

Potassium-sparing diuretics recommended for heart failure include amiloride and triamterene.

Potassium-sparing diuretics are weak when used alone but are often given in combination with thiazide or loop diuretics to prevent low potassium levels in the body, which is known as hypokalaemia.

How do diuretics work?

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

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

In the second stage, your kidneys re-absorb these nutrients and essential substances back into the blood. This leaves waste products, plus some salt and water (urine) in your kidneys. The urine travels down tubes to your bladder where it's stored until you go to the toilet.

Heart failure can make your kidneys re-absorb more water and salt into the blood, and so produce less urine. This is your body's way of trying to compensate for the reduced pumping power of the heart, but it can make matters worse. There is a greater volume of blood for your heart to pump, and so more work for it to do. Also, excess water in the blood can leak out into your lungs, making you feel breathless (pulmonary oedema) and into the legs causing your ankles and feet to swell up (peripheral oedema).

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 and so lose more water from your blood. The overall volume of blood is reduced, which gives your heart less work to do and helps to reduce your blood pressure. This is why diuretics are used to treat heart failure and high blood pressure.

How to take diuretics

Most diuretics are available as tablets, although some are also available as injections. As they usually have an effect for 12 to 24 hours, it's best to take diuretics in the morning or early afternoon so that the extra urine your kidneys produce doesn't interfere with your sleep.

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

Always ask your doctor for advice and read the patient information leaflet that comes with your medicine. Always check with your doctor before taking diuretics if you have liver or kidney problems.

Side-effects of diuretics

As with all medicines, there is a chance that you may get some side-effects with diuretics. However, it's important to remember that if you do get side-effects, you should discuss these with your doctor as he or she may be able to suggest an alternative medicine.

If you take a thiazide diuretic, you might not notice any side-effects if you receive a low dose. Higher doses may give you an upset stomach or make you feel faint or dizzy when you stand up. Thiazide diuretics can upset the balance of minerals in your body, leading to low levels of potassium (hypokalaemia) and sodium (hyponatraemia), and reduced levels of body water (dehydration). If you're taking other medicines for any heart problems, low levels of potassium may cause you to have irregular heartbeats. If you're a man, thiazide diuretics may also cause you to have problems getting an erection.

Side-effects with loop diuretics are similar to those seen with thiazides, although side-effects such as muscle cramps and skin rash may occur. 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 works within one hour and acts for up to six hours - this will increase the number of times you have to get up in the night to go to the toilet.

Interactions of diuretics with other medicines

Check with your doctor or pharmacist before you take any other medicines or herbal remedies at the same time as a diuretic.

Don't take potassium supplements at the same time as potassium-sparing diuretics, unless recommended by your doctor, because this could make the potassium levels in your body too high. Your GP may also suggest that you avoid low-sodium salt substitutes, which can contain high levels of potassium.

Names of common diuretics

Examples of the main types of diuretics are shown in the table.

You may have noticed that your medicine has two or more names. All medicines have a generic name, which is its official medical name. Many medicines also have at least one brand name, which is the trade name. Generic names are written in lower case and brand names start with a capital letter.

Generic names Examples of common brand names

Thiazide diuretics

bendroflumethiazide/bendrofluazide

Aprinox

chlortalidone/chlorthalidone

Hygroton

cyclophenthiazide

Navidrex

indapamide

Natrilix

metolazone

Metenix 5

xipamide

Diurexan

Loop diuretics

furosemide/frusemide

Lasix

bumetanide

Burinex

torasemide

Torem

Potassium-sparing diuretics

amiloride

Amilamont

triamterene

Dytac

Further information

Related topics

Sources

This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.

Publication date: April 2009