Diuretics

Published by Bupa's Health Information Team, March 2011.

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?

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

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

What are the main types of diuretic?

The three most common types of diuretic are:

• thiazides
  
• loop diuretics
  
• potassium-sparing diuretics 

Thiazide diuretics

These medicines are the most commonly prescribed diuretics. Your GP may prescribe you a thiazide diuretic (eg bendroflumethiazide) if you have high blood pressure (particularly if you're over 55 or if you’re of African-Caribbean origin), mild heart failure or severe oedema.

If you have very high blood pressure, you may need to take another medicine with the thiazide. For example, your GP may prescribe an angiotensin-converting enzyme inhibitor, an angiotensin-receptor blocker or a calcium-channel blocker.

Thiazide diuretics can sometimes cause you to lose too much potassium from your body. Potassium has many important roles in your body, including keeping your heart healthy and controlling your blood pressure. 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 heart failure or a condition called pulmonary oedema (which is when you have too much fluid on your lungs).

You may also need some other medicines to take with the loop diuretic if you have high blood pressure, or if other medicines haven’t been effective.

If you’re taking a loop diuretic, you may lose too much potassium from your body. Your doctor 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 prescribe you a potassium-sparing diuretic (eg amiloride) if you have high blood pressure or heart failure.

How do diuretics work?

Diuretics work by interfering with 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 your urine.

Diuretics work by reducing the amount of sodium and water that are re-absorbed 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

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.

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 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.

Special care for elderly people

If you are elderly, you may need to take a lower diuretic dose to begin with. This is because elderly people are more likely to be affected by the side-effects that diuretics cause. Your dose will then be changed according to 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 the healthcare professional who prescribed your medicine before you stop taking it.

General side-effects of diuretics include:

• mild gastro-intestinal problems
• 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 minerals in your body, leading to low levels of potassium (hypokalaemia) and sodium (hyponatraemia)

Less common side-effects of diuretics include:

• certain blood disorders, which can make you more likely to get infections
• male impotence (the inability to achieve or sustain an erection for sexual activity to take place)
• skin rashes

If you’re taking other medicines for any heart problems, low levels of potassium may cause you to have an irregular heartbeat.

This section does not 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

Don’t take potassium supplements at the same time as potassium-sparing diuretics, unless recommended by your GP.

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.

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.

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

For sources and links to further information, see Resources.