An older adrenergic-neurone-blocker for high blood pressure
Guanethidine
An older medicine once used to lower high blood pressure; it is now rarely used.
What is Guanethidine?
Guanethidine is an older blood-pressure medicine in a group called adrenergic neurone blockers, which work on the nerves that control blood vessels. It lowers blood pressure by reducing the nerve signals that keep blood vessels tightened. It is now largely obsolete, having been replaced by newer, better-tolerated treatments. Its most troublesome effects were a marked drop in blood pressure on standing up (causing dizziness or fainting) and diarrhoea. It was taken by mouth under medical supervision.
Education and reference only. This is a plain-language guide to Guanethidine — 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
Guanethidine is an older medicine that was used to treat high blood pressure. It belongs to a group called adrenergic neurone blockers, which act on the nerves that normally tell blood vessels to tighten. It was one of the earlier powerful blood-pressure treatments but has since been overtaken by newer medicines that are easier to tolerate, so it is now largely a historical medicine and rarely used in the UK. It is taken by mouth as a tablet, under medical supervision where used at all.
How it works
Blood vessels are kept partly tightened by signals from certain nerves, which use a chemical messenger (noradrenaline) to do this. Guanethidine works by getting into these nerves and reducing the release of that messenger, so the vessels relax and widen and blood pressure falls. Because this effect is strongest when standing, gravity adds to the drop, which is why it tends to cause a large fall in blood pressure on standing up. Its broad effect on these nerves also explains other effects such as diarrhoea.
Company & origin
Originated / developed by: Generic (long-established).
An older blood-pressure medicine that worked on the nerves controlling blood vessels; it is now largely obsolete.
What it treats
Conditions Guanethidine is used for
Practical use
How to take Guanethidine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth as prescribed, at regular times.
- Stand up slowly from sitting or lying down, as it can cause a large drop in blood pressure on standing.
- Tell your prescriber if you feel very dizzy or faint, especially when getting up.
- Mention diarrhoea or other persistent side effects to your prescriber.
- Do not stop it suddenly without advice, and give a full list of your other medicines.
Weighing it up
Advantages & disadvantages of Guanethidine
Advantages
- Was an effective way of lowering high blood pressure when fewer options existed.
- Taken by mouth as a tablet.
- A long-established medicine with a long history of use behind it.
Disadvantages
- Largely obsolete, having been replaced by newer, better-tolerated blood-pressure medicines.
- Commonly causes a severe drop in blood pressure on standing, with dizziness or fainting.
- Commonly causes diarrhoea and can affect other functions such as sexual function.
Practical use
Good to know
The key thing to know about guanethidine is that it is largely obsolete: newer blood-pressure medicines are safer and far better tolerated, so it is rarely used today. Its most troublesome effect was severe postural hypotension, meaning a large drop in blood pressure when standing up, which could cause marked dizziness or even fainting, so people were advised to stand up slowly. It also commonly caused diarrhoea. Because it acts on the nerves controlling many body functions, it could affect other things too, such as sexual function. As with any blood-pressure treatment, it was not stopped abruptly without advice, and it interacted with several other medicines, so a full medicines list mattered.
Who should not take it / use with caution
- People who have had a serious allergic reaction to guanethidine should not take it.
- It is used with great care, or avoided, in people prone to fainting from low blood pressure on standing.
- It is used with caution in people with certain heart, kidney or other conditions, under medical guidance.
Monitoring
- Checking blood pressure both lying or sitting and standing, to watch for postural drops.
- Reviewing side effects such as dizziness, fainting and diarrhoea.
- Reviewing other medicines for possible interactions.
Side effects
- A marked drop in blood pressure on standing, causing dizziness or fainting.
- Diarrhoea.
- Tiredness or a blocked nose.
- Effects on sexual function in some people.
Key interactions
- Other blood-pressure medicines can add to the drop in blood pressure.
- Some antidepressants and other medicines can block or change its effect, so it interacts with several drugs.
- Tell your prescriber about all your medicines, including those bought without a prescription.
Available as: Tablets taken by mouth.
Answers
Guanethidine: frequently asked questions
What is guanethidine used for?
It is an older medicine that was used to lower high blood pressure by acting on the nerves that keep blood vessels tightened; it is now rarely used.
Is it still commonly used?
No. Guanethidine is largely obsolete and has been replaced by newer blood-pressure medicines that are safer and better tolerated.
Why did it cause dizziness on standing?
It lowers blood pressure most strongly when standing, so gravity adds to the drop, which could cause marked dizziness or even fainting when getting up.
Why does it cause diarrhoea?
It acts broadly on the nerves controlling the body, including the gut, which is why diarrhoea was a common effect.
Can I stop it suddenly?
Blood-pressure medicines should not be stopped abruptly without advice; any changes should be guided by your prescriber.
Authoritative sources
- BNF
- NICE CKS
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