Expert reviewer, Dr Tim Cripps, Consultant Cardiologist
Next review due, June 2022

Diuretics are medicines that increase the amount of urine you produce. This allows your body to get rid of excess water and salts. They’re often called ‘water tablets’.

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Uses of diuretics

Your doctor may prescribe diuretics for you if you have the following.

  • High blood pressure – diuretics are one of four main types of medicines your doctor may prescribe alongside making lifestyle changes.
  • Too much fluid in your body tissues (known as oedema) as a result of heart failure.

They’re sometimes used for other conditions too, such as glaucoma (see our FAQ: Why has my doctor prescribed a diuretic for my glaucoma?)

Diuretics aren’t suitable for everyone. They might make other conditions you have worse, and they can also interact with other medicines you may be taking. They can also be harmful if you’re pregnant. See the Side-effects and Interactions sections below for more information. Always tell your doctor about any other conditions you have, any medicines you’re taking, or if you’re pregnant or breastfeeding.

How do diuretics work?

Your kidneys filter out water, salts and waste products from your blood. Most of the water and salts are reabsorbed back into your bloodstream, but some are left with the waste products to form urine.

Diuretics act by stopping salts and water from being reabsorbed back into your bloodstream, so more is passed in your urine. This makes you need to pee more often, which is why they’re often called ‘water tablets’.

Getting rid of extra salts and fluid in this way decreases the amount in your bloodstream and reduces the overall volume of your blood. This lowers your blood pressure and gives your heart less work to do.

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Types of diuretics

There are different types of diuretics, which have different uses depending on how and where they work. The most common ones are as follows.

  • Thiazide and thiazide-related diuretics. These are mainly used at low doses to treat high blood pressure, but can also be used in higher doses to treat heart failure. They include chlortalidone (eg Hygroton), indapamide (eg Indipam, Natrilix) and bendroflumethiazide.
  • Loop diuretics. These are used to treat a build-up of fluid (oedema) due to heart failure or chronic kidney disease. Examples include furosemide (eg Frusol) and bumetanide. Sometimes they’re used in combination with other medicines to treat high blood pressure.
  • Potassium-sparing diuretics. These diuretics treat fluid build-up due to heart failure, while preventing too much of a particular salt called potassium being passed from your body in your urine. Examples include amiloride, triamterene, spironolactone and eplerenone. They are weak diuretics on their own and so are given with another diuretic.

Taking diuretics

You take most diuretics as tablets that you swallow. Diuretics make you need to pee more often, and usually start to act within one to two hours of taking them. It’s worth thinking about the best time to take them, so that they cause the least disruption to your day. Taking them early in the day might mean that they’re less likely to interfere with your sleep; but this doesn’t work for everyone. Talk to your doctor to work out what will suit you best.

Sometimes, you may have a diuretic as an injection into a vein in your arm. You’ll have this in hospital if you need treatment quickly.

If you forget to take your usual dose of diuretic, take it as soon as you remember. If it’s nearly time for your next dose though, just take that one as normal – don’t take double. If you accidentally take too much, contact your doctor or pharmacist straightaway for advice.

Some diuretics can cause you to lose too much potassium. To help keep your potassium levels healthy, your GP may prescribe you a potassium-sparing diuretic or a potassium supplement, in addition to your usual diuretic.

Don’t stop taking your diuretics unless your doctor tells you to. Without them, your blood pressure would quickly increase.

Always read the patient information leaflet that comes with your medicine and ask your pharmacist or GP if you need any more advice.

Interactions of diuretics


Diuretics can sometimes interact with certain other medicines. This can have an effect on how well your medicines work, or possibly make side-effects more likely.

There will be a list of medicines potentially affected on the patient information leaflet that comes with your medicine, and these may include:

  • digoxin, which is used for various heart conditions
  • antibiotics (eg vancomycin)
  • beta-blockers (eg sotalol)
  • anti-arrhythmics (eg flecainide)
  • non-steroidal anti-inflammatory drugs (NSAIDs) (eg ibuprofen)

Your pharmacist or doctor will be able to tell you for sure if any medicines you take may be affected, and may adjust your dose or suggest an alternative if necessary. Don’t stop any medicine without talking to your doctor first.

Food and drink

It’s important not to eat too much salt if you’re taking a diuretic. Too much salt in your diet can counteract the effects of the diuretics. You should also steer clear of salt substitutes, which are high in potassium if you’re taking potassium-sparing diuretics, otherwise your potassium level may rise too high.

Be careful when drinking alcohol while taking diuretics. Alcohol has a diuretic effect of its own and this may cause your blood pressure to fall too low.

Vitamin and mineral supplements

It’s important not to take potassium supplements at the same time as potassium-sparing diuretics. This is because both of these act to increase the level of potassium in your blood. Taking the two medicines together means your level could become dangerously high (hyperkalaemia).

You may also need to be careful if you take calcium supplements. This is because diuretics can increase the level of calcium in your body. The resulting high levels of calcium can cause various side-effects.

It’s important that you tell your pharmacist or doctor about all the medicines or supplements you’re taking – both prescription and non-prescription. They can then advise you about what interactions and side-effects they may have with a diuretic. Always read the patient information leaflet that comes with your medicine too.

Side-effects of diuretics

Side-effects are unwanted effects that you may get from taking a medicine. There are many different types of diuretic, and each one has different potential side-effects. The best way to find out is to read the patient information leaflet that comes with your medicine or speak to your pharmacist.

Common problems include low blood pressure when standing up, and losing too much potassium or sodium, which can cause muscle weakness, cramps and confusion. Once you’ve started taking a diuretic, you’ll have regular health checks to monitor your blood pressure, kidney function and salt and potassium levels. These may be fairly frequent to begin with; how often will depend on which particular diuretic you’re taking. Eventually you’ll need these checks less often – around every six months to a year.

Some diuretics can worsen or increase your risk of developing diabetes and gout.

Taking diuretics can also cause you to become dehydrated. If you’re ill with diarrhoea and vomiting, you may need to temporarily stop taking your diuretic or increase your fluid intake, to prevent dehydration. Always ask your doctor or pharmacist for advice first.

If you’re having side-effects, give your new medicine a chance and wait a few days to see if they ease. If they continue and you’re feeling unwell or concerned, contact your doctor or pharmacist for advice and help.

Medicines checklist

Our handy medicines checklist helps you see what to check for before taking a medicine.

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Bupa medicines checklist

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Related information

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  • Reviewed by Pippa Coulter, Freelance Health Editor, June 2019
    Expert reviewer Dr Tim Cripps, Consultant Cardiologist
    Next review due June 2022