Cardiovascular / Renal
Loop diuretics
e.g. furosemide — Strong "water tablets" used mainly to relieve fluid overload in heart failure.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Loop diuretics such as furosemide are the main drugs used to remove excess fluid in heart failure and other oedema states. They act quickly and powerfully and are titrated to the patient's fluid status.
How it works
They block sodium and chloride reabsorption in the loop of Henle in the kidney, causing the body to excrete more salt and water. This relieves congestion in the lungs and tissues but can also lower potassium, sodium and blood pressure if overdone.
In practice
In practice loop diuretics are titrated to the patient, not the prescription: more for congestion, less once dry, guided by weight, symptoms and electrolytes. Over-diuresis causes dehydration, low potassium and acute kidney injury, so check bloods after changes. Teaching patients to track daily weights and recognise a quick gain helps catch fluid building up before it becomes an admission.
Examples
Practical use
How to take it & use it well
- Take it in the morning, and if a second dose is needed take it by early afternoon, so you are not woken at night needing the toilet.
- Expect to pass more urine within a few hours of taking it, so plan around easy access to a toilet.
- Take it at the same time each day and weigh yourself regularly if advised, as this helps track fluid levels.
- If you miss a dose, take it when you remember if it is still early in the day, but skip it if it is late, to avoid being up at night. Do not double up.
- Stand up slowly, as these medicines can lower blood pressure and cause dizziness, especially in hot weather or if you are dehydrated.
- Keep up with the blood tests your doctor arranges to check your kidneys and salts such as potassium.
Common uses
- Fluid overload in heart failure
- Oedema in renal and liver disease
- Acute pulmonary oedema (urgent setting)
Monitoring
- Renal function and electrolytes (especially potassium and sodium)
- Weight and fluid status
- Blood pressure
Weighing it up
Advantages & disadvantages
Advantages
- They are very effective at removing excess fluid, easing swelling in the legs and lungs.
- They relieve breathlessness and swelling in heart failure, often quite quickly.
- They can be adjusted up or down to match how much fluid needs removing.
- They are useful in kidney problems where milder water tablets do not work well.
- They are well established and inexpensive on the NHS.
Disadvantages
- They can lower important salts such as potassium and sodium, which needs blood-test monitoring.
- They can cause dehydration, dizziness and low blood pressure if too much fluid is lost.
- They make you pass urine frequently, which can disrupt daily life and sleep if taken late.
- They can affect kidney function and raise the risk of gout.
- They can cause cramps and, with high doses over time, occasionally affect hearing.
Key safety principles
What to watch for
- Can cause dehydration, low potassium and sodium, and acute kidney injury if over-diuresed.
- Additive effects with ACE inhibitors and other antihypertensives can drop blood pressure.
- Ototoxicity at high doses, particularly given quickly or with other ototoxic drugs.
Key interactions
What to avoid or check alongside
- Combining them with aminoglycoside antibiotics can increase the risk of hearing damage.
- Anti-inflammatory painkillers (NSAIDs) can reduce their effect and increase the risk of kidney problems.
- Taking them with ACE inhibitors or ARBs can cause a sharp drop in blood pressure, especially with the first dose.
- They can lower potassium, which is risky when combined with digoxin, a heart medicine.
- Other medicines that lower potassium, such as steroids, add to this effect.
- Lithium levels can rise when fluid is lost, needing extra monitoring.
Patient & carer advice
- Take in the morning to avoid night-time trips to the toilet
- Weigh yourself regularly if advised — a quick rise can signal fluid building up
- Report dizziness, cramps or a marked drop in how much you pass urine
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Loop diuretics: frequently asked questions
When should I take it?
Take it in the morning so the increased need to pass urine happens during the day. If a second dose is needed, take it by early afternoon to avoid being up at night.
Why do I need blood tests?
These medicines can lower salts like potassium and affect the kidneys. Blood tests make sure your levels stay safe and the dose is right for you.
Can I drink less water to pass less urine?
No. Restricting fluids can cause dehydration. Follow any fluid advice from your team, and take the medicine as prescribed rather than changing how much you drink on your own.
Can I drink alcohol with it?
Alcohol can add to the drop in blood pressure and dizziness and can worsen dehydration, so it is best kept moderate.
Is it safe in pregnancy?
Loop diuretics are not routinely used in pregnancy and are only given for specific reasons under specialist care. Tell your doctor if you are pregnant or planning to be.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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