Loop diuretics are commonly used in the treatment of heart failure. In this condition, fluid accumulates in your body, due to the heart not pumping blood around the body as well as it normally would. So, you may become breathless (as fluid accumulates in the lungs) and your ankles and legs may swell with extra fluid in the tissues (oedema).
Loop diuretics are also used to treat other conditions which cause fluid to build up in the body, such as certain liver and kidney disorders. They are also sometimes used to treat high blood pressure (hypertension). However, generally, a different type of diuretic called a thiazide diuretic is more commonly used to treat high blood pressure.
How do loop diuretics work?
They work by making the kidneys pass out more fluid. They do this by interfering with the transport of salt and water across certain cells in the kidneys. (These cells are in a structure called the loop of Henle - hence the name loop diuretic. You have thousands of these loops in each kidney.) As more fluid is passed out by the kidneys, less fluid remains in the bloodstream. So any fluid which has accumulated in the tissues of the lungs or body is drawn back into the bloodstream to replace the fluid passed out by the kidneys. This eases symptoms such as oedema and breathlessness caused by the congestion of fluid.
What are the possible side-effects?
Side-effects are uncommon when routine low doses are used. The higher the dose, the greater the risk of side-effects developing. The leaflet which comes in the tablet package provides a full list of possible side-effects. So, it is important to read that leaflet if you are prescribed a loop diuretic. Common or serious possible side-effects include:
- The salt balance in the bloodstream sometimes being upset, which can cause a low blood level of potassium, sodium, and magnesium, and a high level of calcium. These effects may cause weakness, confusion and, rarely, abnormal heart rhythms to develop. You may be advised to have a blood test to check for these problems.
- If you have diabetes or gout, these conditions may be made worse by diuretics.
- An upset stomach
- Dizziness on standing - due to too low blood pressure (hypotension).
Most loop diuretics are taken once a day in the morning. The effect of making more urine usually starts within half an hour. So, you may find you need to go to the toilet a couple of times within a few hours of taking the tablet. However, the effect of passing extra urine wears off within about six hours. So, you will not have to get up in the night to make extra trips to the toilet.
You can generally take the dose at a flexible time to suit you. So, for example, if you want to go out early in the morning for a few hours and don't want to have to find a toilet, you may be able to postpone the dose until later on in the day when it may be more convenient to find toilets.
How to use the Yellow Card Scheme
If you think you have had a side-effect to one of your medicines you can report this on the Yellow Card Scheme. You can do this online at www.mhra.gov.uk/yellowcard.
The Yellow Card Scheme is used to make pharmacists, doctors and nurses aware of any new side-effects that medicines or any other healthcare products may have caused. If you wish to report a side-effect, you will need to provide basic information about:
- The side-effect.
- The name of the medicine which you think caused it.
- The person who had the side-effect.
- Your contact details as the reporter of the side-effect.
It is helpful if you have your medication - and/or the leaflet that came with it - with you while you fill out the report.
Further reading and references
Hypertension in adults: diagnosis and management; NICE (August 2019 - last updated March 2022)
Chronic heart failure in adults - diagnosis and management; NICE Guidance (Sept 2018)
British National Formulary (BNF); NICE Evidence Services (UK access only)
Oh SW, Han SY; Loop Diuretics in Clinical Practice. Electrolyte Blood Press. 2015 Jun13(1):17-21. doi: 10.5049/EBP.2015.13.1.17. Epub 2015 Jun 30.