An alpha-blocker
Prazosin
An alpha-blocker used for high blood pressure and for an enlarged prostate.
What is Prazosin?
Prazosin is an alpha-blocker, a medicine that relaxes the muscle in blood vessel walls and in the prostate. By relaxing blood vessels it lowers blood pressure, and by relaxing the prostate and bladder outlet it eases the urinary symptoms of an enlarged prostate. Its best-known caution is the "first-dose effect": the very first dose can cause a sudden drop in blood pressure with dizziness or fainting, so it is taken at bedtime to start. Getting up slowly helps avoid feeling faint.
Education and reference only. This is a plain-language guide to Prazosin — 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
Prazosin is an alpha-blocker, a medicine that blocks a signal which keeps muscle in blood vessel walls and the prostate tightened. By relaxing those muscles it widens blood vessels to lower blood pressure and eases the flow of urine in men with an enlarged prostate (benign prostatic hyperplasia). It is taken by mouth as a tablet, usually more than once a day. The first dose is treated with care because of its effect on blood pressure.
How it works
Prazosin blocks alpha receptors, which normally receive signals telling the muscle in blood vessel walls and the prostate to tighten. With these signals blocked, the blood vessels relax and widen, lowering blood pressure, and the muscle around the prostate and bladder outlet relaxes, making it easier to pass urine. Because the first dose can relax the vessels suddenly, blood pressure can drop sharply at first, which is why the first dose is taken at bedtime.
Company & origin
Originated / developed by: Generic (long-established).
A long-established alpha-blocker used in the UK for high blood pressure and for prostate-related urinary symptoms.
What it treats
Conditions Prazosin is used for
Practical use
How to take Prazosin
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take the very first dose at bedtime to reduce the risk of a sudden drop in blood pressure.
- Take it regularly as prescribed thereafter, usually more than once a day.
- Get up slowly from sitting or lying down, especially when you first start or after a dose increase.
- If you feel faint, sit or lie down until it passes, and tell your prescriber if it keeps happening.
- Take extra care if you also take water tablets or other blood pressure medicines, which add to the effect.
- Do not stop it suddenly without advice, particularly if you take it for blood pressure.
Weighing it up
Advantages & disadvantages of Prazosin
Advantages
- Lowers blood pressure and, in men, eases the urinary symptoms of an enlarged prostate.
- A long-established medicine with familiar effects.
- Relaxes blood vessels and the prostate without affecting the heart rate directly.
Disadvantages
- The first dose can cause a sudden drop in blood pressure with dizziness or fainting.
- Can cause ongoing faintness on standing, drowsiness and a stuffy nose.
- Usually needs to be taken more than once a day.
Practical use
Good to know
The most important thing to know about prazosin is the "first-dose effect": the first dose, or the first dose after an increase, can cause a sudden fall in blood pressure with dizziness, light-headedness or even fainting. To reduce this, the first dose is taken at bedtime, and it helps to get up slowly afterwards. Feeling faint on standing can continue to some degree, especially early on or if you are also taking water tablets. Other common effects include drowsiness, a stuffy nose and headache. In men it is also used to relieve the urinary symptoms of an enlarged prostate.
Who should not take it / use with caution
- People who have fainting when passing urine should avoid it, as it can worsen this.
- Used with caution in those prone to feeling faint on standing, including some older people.
- Care is needed in people having cataract surgery, as alpha-blockers can affect the eye during the operation.
Monitoring
- Blood pressure checks, including for faintness on standing, especially when starting or increasing the dose.
- Reviewing dizziness or fainting and whether the timing of doses helps.
- In men, checking that prostate-related urinary symptoms are improving.
Side effects
- Dizziness, light-headedness and fainting, especially with the first dose or on standing.
- Drowsiness, headache, a stuffy nose and tiredness.
- Swollen ankles or, in men, occasionally problems with ejaculation.
Key interactions
- Adds to the blood-pressure-lowering effect of other blood pressure medicines and water tablets.
- Medicines for erectile problems can together cause a marked drop in blood pressure.
- Care is needed with other medicines that cause faintness on standing, so tell your prescriber what you take.
Available as: Tablets taken by mouth.
Answers
Prazosin: frequently asked questions
Why should I take the first dose at bedtime?
The first dose can cause a sudden drop in blood pressure with dizziness or fainting, so taking it at bedtime means you are lying down if this happens.
What is the first-dose effect?
It is a sharp fall in blood pressure that can occur with the first dose, or the first dose after an increase, causing dizziness, light-headedness or fainting.
Why do I feel faint when I stand up?
Prazosin relaxes blood vessels, which can make blood pressure drop on standing; getting up slowly helps, and you should tell your prescriber if it persists.
Can it help my prostate symptoms?
Yes, in men it relaxes the muscle around the prostate and bladder outlet, easing the urinary symptoms of an enlarged prostate.
Can I take it with tablets for erection problems?
Together they can cause a marked drop in blood pressure, so tell your prescriber, who will advise how to take them safely.
The wider class
About Alpha-blocker
Prazosin belongs to the alpha-blocker 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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