Diuretics

Your health expert: Dr Joshua T Y Chai, Consultant cardiologist
Content editor review by Dr Kristina Routh, August 2021
Next review due August 2024

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’.

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.
Too much fluid in your body tissues (known as oedema) as a result of heart failure.

They’re sometimes used for other conditions too. These include glaucoma, where a type of diuretic may be given as eye drops to help lower the pressure in your eye.

Diuretics aren’t suitable for everyone. They might cause problems if you have certain other medical conditions. And they can also interact with other medicines you may be taking. See the Side-effects and Interactions sections below for more information. Always tell your doctor about any other conditions you have and any medicines you’re taking.

Make sure your doctor knows if you are, or may be, pregnant. Some diuretics can harm your unborn baby. If you’re breastfeeding, ask your doctor whether your medicine could cause problems for your baby.

How do diuretics work?

Your kidneys filter out water, salts and waste products from your blood. Most of the water and salts are then taken back into your bloodstream. But some are left with the waste products to form urine (pee).

Diuretics stop salts and water from being taken back into your bloodstream. This means more water and salts are passed in your pee. 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 lowers the amount in your bloodstream. This reduces the volume of your blood. This helps lower your blood pressure and gives your heart less work to do.

Types of diuretics

There are different types of diuretics which have different uses. The most common ones are described here.

Thiazide and thiazide-related diuretics. These are mainly used at low doses to treat high blood pressure. But they may also be used to treat heart failure. An example is bendroflumethiazide.
Loop diuretics. These are used to treat a build-up of fluid in heart failure or chronic kidney disease. An example is furosemide. Sometimes they’re used together with other medicines to treat high blood pressure.
Potassium-sparing diuretics. Like other diuretics, these make you pass out more water. But they stop you losing too much of a salt, called potassium, at the same time. They’re usually used with other diuretics to stop your blood potassium going too low. An example is amiloride.

Over-the-counter, or ‘natural’ diuretics

The diuretic medicines listed above must all be prescribed by your doctor. But you may have heard of diuretics you can buy over the counter. These are sometimes called ‘natural diuretics’. These are herbs or dietary supplements that claim to have a mild diuretic effect and so treat ‘water retention’. Some people use them for water retention linked to their periods. But there’s no evidence to show that they work.

If you think you may have water retention, talk to your GP first rather than trying to treat it yourself. A number of serious medical conditions can make your body hold on to too much water. And some herbal remedies and supplements can cause problems with other medicines you’re taking. Your pharmacist can give you advice about these.

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 cause problems with your sleep. Talk to your doctor to work out what suits you best.

If you’re taking diuretics and you have a long journey to make, you may need to think about planning some breaks because you’ll need to pee more often.

If you need treatment quickly, you may have a diuretic as an injection into a vein in your arm. 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 right away for advice.

Some diuretics can cause you to lose too much potassium. So, alongside your usual diuretic your GP may prescribe either:

a potassium-sparing diuretic (see ‘types of diuretic’ for more information about these)
a potassium supplement

Always read the patient information leaflet that comes with your medicine. If you need advice about taking your medicines, ask your pharmacist.

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Interactions of diuretics

Medicines

Diuretics can sometimes interact with certain other medicines. This can affect how well your medicines work, or make side-effects more likely.

The patient information leaflet that comes with your diuretic has a list of medicines that might interact. These include other medicines you may be taking for high blood pressure or heart failure. It’s important to read this list. Diuretics may also interact with medicines you buy over the counter for pain relief, such as ibuprofen.

Your pharmacist or doctor will be able to tell you for sure if any medicines you take may be affected. Make sure your doctor knows what you’re already taking when they prescribe you medicines. Your doctor may adjust your dose or suggest an alternative if needed. Don’t stop any medicine without talking to your doctor first.

Food and drink

Too much salt in your diet can counteract the effects of some diuretics. So you might have to reduce the amount of salt you eat. You should always follow your doctor’s advice and read the information that comes with your medicine. Ask your doctor if you’re not sure how much salt you can eat.

If you’re taking potassium-sparing diuretics, you should avoid salt substitutes which are high in potassium. Otherwise, your potassium level may become too high.

Be careful when drinking alcohol while taking diuretics. Alcohol and diuretics together may make your blood pressure fall too low.

Vitamin and mineral supplements

It’s important not to take potassium supplements at the same time as potassium-sparing diuretics. Taking the two medicines together could make your potassium level dangerously high.

You may also need to be careful if you take calcium supplements. Taken together, calcium supplements and some diuretics can make the level of calcium in your body go too high. Make sure your doctor knows if you’re taking any supplements when they’re prescribing medicines. 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 diuretics, and each can have different side-effects. The best way to find out is to read the patient information leaflet that comes with your medicine. Or ask your pharmacist.

Common problems of diuretics include:

dizziness
feeling very tired
headache
feeling sick
low blood pressure, especially when standing up
losing too much potassium or sodium, which can cause muscle weakness and cramps

Diuretics can sometimes affect other medical conditions you may have. If you have gout, some diuretics can make your symptoms worse. If you have diabetes, they can make your blood sugar level higher.

Taking diuretics can also make you dehydrated, which means you don’t have enough water in your body. Your doctor can advise you about how much fluid to drink. You should contact your doctor if you feel thirsty, lightheaded or dizzy while on diuretics. These may be signs of dehydration. If you have diarrhoea and vomiting, you may need to stop taking your diuretic for one to two days. This will help you avoid dehydration. If your symptoms go on for more than two days, contact your GP.

In very hot weather, or if you travel to a hot country, you’re more at risk of dehydration. Monitor the colour of your pee to give you an idea of how hydrated you are. But if you’re on high doses of diuretics, you may also need to talk to your GP.

Image showing hydration level by urine colour

If you’re having mild side-effects, wait a few days to see if they ease. If they continue and you’re feeling unwell or concerned, contact your doctor or pharmacist.

Check-ups

Once you’ve started taking a diuretic, you’ll have regular health checks. These monitor your blood pressure, kidney function and salt and potassium levels. Checks may be fairly frequent to begin with. How often will depend on which diuretic you’re taking. Eventually you’ll need these checks less often – around every six months to a year.

Medicines checklist

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

Bupa's medicines checklist PDF opens in a new window (0.8MB)

Diuretics are medicines that increase the amount of urine (pee) you produce. This allows your body to get rid of excess water and salts. They’re often called water tablets. Your doctor may prescribe diuretics for you if you have high blood pressure or heart failure. See our sections on ‘Uses of diuretics’ and ‘How diuretics work’ for more information.

There are a number of different types of diuretics. The main types your doctor may prescribe are:

thiazide and thiazide-related diuretics (eg bendroflumethiazide)
loop diuretics (eg furosemide)
potassium-sparing diuretics (eg amiloride)

To find out more about each, and why and how they’re used, see our section on ‘types of diuretics’.

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