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

Why would I take diuretics?

The most common reasons you may need to take a diuretic are if you have:

  • high blood pressure
  • too much fluid in your body tissues (known as oedema), as a result of heart failure, which is when your heart loses its ability to pump blood efficiently through your body

What are the main types of diuretic?

The three most common types of diuretic are:

  • thiazide and related diuretics
  • loop diuretics
  • potassium-sparing diuretics

Thiazide and related diuretics

Your GP may prescribe you a thiazide or related diuretic (eg indapamide) if you have high blood pressure (particularly if you're over 55 or if you’re of African-Caribbean origin), mild heart failure or oedema.

If your blood pressure isn’t controlled with a thiazide alone, you may need to take another medicine with the thiazide. For example, your GP may prescribe an angiotensin-converting enzyme inhibitor or an angiotensin-receptor blocker. If this still fails to control your blood pressure, your GP may prescribe a calcium channel blocker.

Thiazide diuretics can sometimes cause you to lose too much potassium from your body. Potassium has many important roles, such as to keep your heart healthy and control the balance of fluids in your body. Your doctor may therefore need to give you a potassium supplement or a potassium-sparing diuretic to take at the same time.

Loop diuretics

These medicines are the most powerful type of diuretic. Loop diuretics increase the amount of urine your kidneys produce very quickly. Your GP may prescribe you a loop diuretic (eg furosemide) if you have a condition called peripheral oedema, which is when you have too much fluid in your legs.

If you’re taking a loop diuretic, you may lose too much potassium from your body. Your GP may therefore need to give you a potassium supplement or a potassium-sparing diuretic to take as well.

Potassium-sparing diuretics

These medicines are mild diuretics. Potassium-sparing diuretics are usually used together with a thiazide or loop diuretic to stop you losing too much potassium from your body.

Your GP may also prescribe you a potassium-sparing diuretic (eg amiloride) in combination with other medicines if you have high blood pressure or heart failure.

How do diuretics work?

Diuretics work by changing the filtration process in your kidneys.

Your kidneys’ normal function is to filter out water, salts (mainly potassium and sodium) and waste products from your bloodstream. Most of the water and salts are re-absorbed back into your bloodstream, but some is left with the waste products, which form urine.

Diuretics work by reducing the amount of sodium and water that your body re-absorbs back into your bloodstream. This increases the amount of urine your body produces and decreases the amount of fluid left in your bloodstream. The overall volume of your blood is therefore 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

You can take most diuretics as tablets, although some are also available as injections. Thiazide diuretics usually have an effect for 12 to 24 hours, so it’s best to take them in the morning so that the extra urine your kidneys produce doesn’t interfere with your sleep. Loop diuretics complete their effect within six hours so you can have these twice a day if necessary without them disturbing your sleep.

As some diuretics can cause your blood potassium levels to fall, your GP may advise you to take a potassium supplement or eat foods with plenty of potassium in them. Bananas, milk and fish are all good sources of potassium.

Always ask your GP for advice and read the patient information leaflet that comes with your medicine.

Special care for elderly people

If you’re elderly, you may need to take a lower diuretic dose to begin with. This is because you’re more likely to be affected by the side-effects that diuretics cause. Your doctor may then change the dose, depending on how well your kidneys are working.

Side-effects of diuretics

Side-effects are the unwanted effects of taking a medicine. If you have side-effects, it’s important to talk to your doctor or healthcare professional who prescribed your medicine – don’t stop taking it before you speak to him or her.

Side-effects of diuretics include:

  • mild gastro-intestinal problems, such as feeling sick
  • a fall in blood pressure that is related to posture (postural hypotension), which causes you to feel faint or dizzy when you stand up
  • altered levels of salts in your body, such as low levels of potassium (hypokalaemia) and sodium (hyponatraemia)

Less common side-effects of diuretics include:

  • gout (a condition that causes pain and swelling in your joints)
  • impotence in men (the inability to achieve or sustain an erection during sex)
  • skin rashes
  • headaches
  • certain blood disorders, which can make you more likely to get infections

This section doesn't include every possible side-effect of diuretics. Please read the patient information leaflet that comes with your medicine for more information.

Interactions of diuretics with other medicines

It’s important not to take potassium supplements at the same time as potassium-sparing diuretics, unless your GP recommends it.

If you’re taking other medicines for a heart problem, low levels of potassium caused by a diuretic may cause you to have an irregular heartbeat.

For more information about interactions of diuretics, see our frequently asked questions. Check with your GP or pharmacist before you take any other medicines or herbal remedies at the same time as a diuretic.

Names of common diuretics

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

All medicines have a generic name. Many medicines also have one or more brand names. Generic names are in lower case, whereas brand names start with a capital letter.

Generic names Examples of common brand names
Thiazide and related diuretics  
bendroflumethiazide Aprinox
chlortalidone/chlorthalidone Hygroton
cyclophenthiazide Navidrex
indapamide Natrilix
metolazone (Non-proprietary)
xipamide Diurexan
Loop diuretics  
furosemide/frusemide Lasix
bumetanide (Non-proprietary)
torasemide Torem
Potassium-sparing diuretics  
amiloride Amilamont
triamterene Dytac


Produced by Rachael Mayfield-Blake, Bupa Heath Information Team, November 2012.

For answers to frequently asked questions on this topic, see FAQs.

For sources and links to further information, see Resources.

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  • This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the about our health information page.

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