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Diuretics are medicines that are often used to treat high blood ­pressure. There are different types of diuretic, but the type most commonly used to treat high blood pressure are called “thiazide” diuretics. Most of these medicines have names that end in “ide”.

All about diuretics

How do diuretics work?

Diuretics are sometimes called ‘water tablets’ ­because they can cause you to pass more urine than usual. They work on your kidneys by ­increasing the amount of salt and water that comes out through your urine. Too much salt can cause extra fluid to build up in your blood vessels, raising your blood pressure. Diuretics lower your blood pressure by flushing salt out of your body, taking this unwanted extra fluid with it.

Diuretics also cause the walls of your blood vessel to relax and widen, which making it easier for your blood to flow through. This effect also lowers your blood pressure.

Who might be given a diuretic?

Most people who have high blood pressure will need to take one or more medicines to control it. Thiazide diuretics are a popular first-choice ­medicine for high blood pressure.

They are particularly recommended for people who are over 55 or who are of African Caribbean origin. However, even if neither of these apply to you, you may be given a thiazide diuretic as a first choice. If you need to take more than one ­medicine to control your blood pressure enough, you may be given a diuretic as a second or third medicine.

Other types of diuretic, called “loop diuretics” or “potassium-sparing diuretics”, may sometimes be given to people who cannot take a thiazide, or to people already taking three blood pressure ­medicines. Potassium-sparing diuretics may be used in people who have low potassium levels, ­because other diuretics can cause a drop in potassium levels. 

Who would not take a diuretic?

Rarely, diuretics can interfere with other health ­problems you may have, or with medicines you are taking. For example, if you have urinary ­problems, gout, severe kidney or liver disease, or Addison’s ­disease (a rare condition affecting the adrenal glands) you should not be given a thiazide  diuretic. Your doctor should always consider your overall health and treatment, not just blood ­pressure treatment, before giving you any new medicine.

Some people who are given a diuretic will need to be monitored closely. These include more elderly people, people with kidney or liver problems, ­people with diabetes, or people who have low potassium or sodium levels.

Diuretics used in the UK

Medicine name Brand names 
BendroflumethiazideAprinox, Bendroflumethiazide, Bendroflumethiazide BP, Prestim
Chlortalidone Co-Tenidone, Co-Tenidone BP, Hygroton, Tenoret 50, Tenoretic 
Cyclopenthiazide Navidrex 
Indapamide Coversyl Plus, Natrilix, Natrilix SR 
Metolazone Metenix 5 
Xipamide Diurexan 
Bumetanide Bumetanide, Burinex 
Furosemide Furosemide, Lasix 
Torasemide Torasemide, Torem 
Eplerenone Inspra 
Spironolactone Idactone, Co-flumactone, Lasilactone, ­Spironolactone 
Triamterene Frusene, Co-triamterzide, Dyazide, Dytide, Kalspare 
Amiloride Amilamont, Amiloride, Burinex, Co-Amilofruse, Co-amilozide, Frumil, Frumil LS, Moduret 25, Moduretic 

Do diuretics have side-effects?

Most people will have no side-effects from their blood pressure medicines. They usually only ­happen when you start to take a new medicine, or a higher dose of your medicine. If you do ­experience a side-effect it may lessen over time as your body gets used to the medicine. If not, your doctor may change your dosage or your medicine if they feel it is appropriate.

Possible side-effects of ­diuretic include:

  • an increased need to go to the toilet
  • feeling thirsty, dizzy, weak, lethargic or sick
  • low blood pressure when moving from lying or sitting to standing
  • muscle cramps
  • skin rash
  • raised uric acid levels (that can lead to kidney problems and gout)
  • raised blood sugar levels
  • problems with erections in men (rarely).

Diuretics can also lower the amount of potassium in your body. This may put you at risk of a ­condition called hypokalaemia, which can be dangerous to your health. You should have your potassium levels monitored by your doctor or nurse if you are taking a thiazide or loop (but not a potassium-sparing) diuretic.

Other things to be aware of when taking diuretics

Many doctors will recommend that you take your­ ­diuretic in the morning as it is likely to cause you to produce more urine than normal. This may help you avoid having to get up in the night to go to the toilet.

Some people taking a diuretic may need to have regular blood and urine tests to check potassium and blood sugar levels. This is to make sure that all the chemicals in your blood stream are properly balanced.

Taking a thiazide diuretic with a beta-blocker can ­increase your long-term risk of developing ­diabetes. If you are taking both these types of medicines ­together, you may wish to speak to your doctor or pharmacist about whether this is right for you.

Check with your doctor or pharmacist before ­taking any other medicines in combination with your ­diuretic – this includes over-the-counter treatments.

As with other blood pressure lowering medicines, once you start taking a diuretic, be prepared to stay on it for the long term. Some people stop ­taking their blood pressure medicines when they think their blood pressure is under control, but doing this can put your health at risk.  If you have any concerns about your diuretic, speak to your doctor or pharmacist.

Published May 2009

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