A thiazide 'water tablet' for blood pressure and fluid
Chlorothiazide
A thiazide 'water tablet' used to lower high blood pressure and reduce fluid retention.
What is Chlorothiazide?
Chlorothiazide is a thiazide diuretic, often called a 'water tablet', used to lower high blood pressure and to reduce fluid retention (swelling) caused by problems such as heart failure. It works by helping the kidneys pass out extra salt and water. Common effects to watch for are low potassium and low sodium, raised blood sugar and raised uric acid (which can trigger gout), and dehydration, so blood tests are checked from time to time. It is related to sulfonamide medicines, so allergy to those matters.
Education and reference only. This is a plain-language guide to Chlorothiazide — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Chlorothiazide is one of the thiazide diuretics, a group of medicines commonly known as 'water tablets'. It is used to lower high blood pressure and to reduce the swelling that builds up when the body holds on to too much fluid, for example in heart failure. It works on the kidneys to help the body get rid of extra salt and water. It is an older member of this group; in the UK other thiazide-type tablets are used more often, but it works in the same general way. It is taken by mouth.
How it works
Chlorothiazide acts on the kidneys to stop them reabsorbing some of the salt (sodium) from the urine. Where salt goes, water follows, so more salt and water are passed out, which lowers the volume of fluid in the body and helps reduce blood pressure and swelling. Over time it also relaxes blood vessels a little, which adds to its blood-pressure-lowering effect. Because it changes how the kidneys handle salts, it can lower potassium and sodium and affect other body chemicals, which is why blood tests are used to keep an eye on the balance.
Company & origin
Originated / developed by: Generic (long-established).
An older thiazide-type diuretic used to lower blood pressure and reduce fluid build-up, related to more commonly used thiazides in the UK.
What it treats
Conditions Chlorothiazide is used for
Practical use
How to take Chlorothiazide
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth as prescribed, usually in the morning so you are not up at night passing water.
- If you take a second dose, take it earlier in the afternoon rather than at bedtime.
- Keep up with the blood tests you are offered to check your kidneys, salts and sometimes sugar.
- Stand up slowly, especially at first, as it can make you feel dizzy or lightheaded.
- Tell your prescriber if you become unwell with vomiting, diarrhoea or are unable to drink, as this can lead to dehydration.
Weighing it up
Advantages & disadvantages of Chlorothiazide
Advantages
- An effective, long-established way to lower high blood pressure and reduce fluid retention.
- Taken by mouth as a simple once-daily type of tablet for many people.
- Often works well alongside other blood-pressure medicines.
Disadvantages
- Can lower potassium and sodium and cause dehydration, so blood tests are needed.
- Can raise blood sugar and raise uric acid, which may trigger gout.
- Means passing more water, so it is best avoided late in the day.
Practical use
Good to know
Because chlorothiazide makes you pass more water, it is usually taken earlier in the day so it does not disturb your sleep with night-time trips to the toilet. The main things to be aware of are the changes it can cause in the body's chemistry: it can lower potassium and sodium, raise blood sugar, and raise uric acid, which can bring on an attack of gout in people prone to it. It can also cause dehydration and dizziness, especially when standing up, particularly in hot weather or if you are unwell with vomiting or diarrhoea. Blood tests are used to check your kidneys, salts and sometimes sugar. It is related to sulfonamide medicines, so tell your prescriber if you have ever reacted to a 'sulfa' medicine.
Who should not take it / use with caution
- People who have had a serious allergic reaction to chlorothiazide or to sulfonamide ('sulfa') medicines should not take it.
- It is used with care, or avoided, in people with severe kidney or liver problems, or with very low potassium or sodium.
- It is used with caution in people with gout, diabetes or certain other conditions, under medical guidance.
Monitoring
- Blood tests to check kidney function and blood salts such as potassium and sodium.
- Blood pressure checks to see how well it is working.
- Checking blood sugar and uric acid where relevant, especially in people with diabetes or gout.
Side effects
- Passing more water, especially soon after starting, and feeling thirsty.
- Dizziness or lightheadedness, particularly when standing up.
- Changes in blood salts such as low potassium or low sodium, sometimes causing tiredness, cramps or weakness.
- Raised blood sugar or raised uric acid, which can bring on gout in those prone to it.
Key interactions
- Taken with other blood-pressure medicines it can lower blood pressure further, sometimes causing dizziness.
- Medicines that also affect potassium, and some heart medicines such as digoxin, need careful monitoring.
- Anti-inflammatory painkillers (NSAIDs) and lithium can interact with it, so tell your prescriber what you take.
Available as: Tablets and an oral liquid taken by mouth.
Answers
Chlorothiazide: frequently asked questions
What is chlorothiazide used for?
It is a thiazide 'water tablet' used to lower high blood pressure and to reduce fluid retention, such as the swelling seen in heart failure.
Why should I take it in the morning?
It makes you pass more water, so taking it earlier in the day helps avoid being woken at night by trips to the toilet.
Why do I need blood tests?
It can change the body's salts, such as lowering potassium and sodium, and affect blood sugar and uric acid, so blood tests help keep these in balance.
Can it bring on gout?
It can raise uric acid levels, which may trigger an attack of gout in people who are prone to it, so tell your prescriber if you have had gout.
Does a sulfa allergy matter?
Yes. Chlorothiazide is related to sulfonamide medicines, so tell your prescriber if you have ever had a serious reaction to a 'sulfa' medicine.
The wider class
About Thiazide diuretic ('water tablet')
Chlorothiazide belongs to the thiazide diuretic ('water tablet') class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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