Prazosin - an alpha-blocker

  • Take your first dose of prazosin at bedtime. If you feel dizzy or faint, stay lying down until the feeling passes.
  • As your body is getting used to prazosin you may feel dizzy or light-headed when standing or sitting up quickly - getting up more slowly will help.
  • Do not drive or use tools or machines if you feel tired or dizzy.
Type of medicineAlpha-blocker
Used forEnlargement of the prostate gland in men
Hypertension (high blood pressure)
Heart failure
Raynaud's syndrome
Also calledHypovase®
Available asTablets

Prazosin belongs to a group of medicines known as alpha-blockers. It works by blocking the action of certain nerve impulses. This blocking action can be used to help control a variety of different medical conditions.

The prostate gland commonly becomes larger in older men. Prostate gland enlargement is also called benign prostatic hyperplasia (BPH). It can cause problems with passing urine, such as having to wait before your urine starts to flow, taking longer at the toilet, dribbling, and a feeling that your bladder is not quite empty. Prazosin works by relaxing the muscles around your bladder and prostate so that you can pass urine more easily.

Prazosin works in high blood pressure and heart failure by relaxing blood vessels. This allows blood and oxygen to circulate more freely around your body, lowering blood pressure and reducing strain on your heart.

In Raynaud's disease, prazosin relaxes the blood vessels in your hands so that blood can reach your fingers more easily. This helps to prevent coldness and stiffness.

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking prazosin it is important that your doctor or pharmacist knows:

  • If you are pregnant, trying for a baby or breast-feeding.
  • If you have liver or kidney problems.
  • If you ever feel dizzy or faint when you stand up, or if you have ever fainted after passing urine.
  • If you are are due to have cataract eye surgery.
  • If you have ever had an allergic reaction to this or to any other medicine.
  • If you are taking any other medicines, including those available to buy without a prescription, herbal and complementary medicines.
  • Before you start this treatment, read the manufacturer's printed information leaflet from inside your pack. The leaflet will give you more information about the specific brand of prazosin you have been given, and a full list of possible side-effects from taking it. Take prazosin exactly as your doctor has told you.
  • Your doctor or pharmacist will tell you how many tablets to take and when to take them. Your dose will also be on the label of your tablets. When first starting this treatment, your doctor will give you a small dose which may then be increased. This allows your doctor to make sure that you have the dose that helps your condition and avoids any unwanted symptoms.
  • Your first dose of prazosin may make you feel faint, so it is important that you take it at bedtime. If you feel dizzy or weary, or if you start sweating, remain lying down until these symptoms have gone.
  • You can take prazosin tablets before, during or after your meals.
  • Try to take your doses at the same times each day. This will help you to remember to take them.
  • If you do forget to take a dose, take it as soon as you remember (unless it is nearly time for your next dose, in which case leave out the missed dose). Do not take two doses together to make up for a forgotten dose.
  • Prazosin can cause dizziness particularly when you first start taking it. Make sure your reactions are normal before you drive or do things which would be dangerous if you were not fully alert.
  • You are advised not to drink alcohol while you are on prazosin. Alcohol will increase the side-effects of prazosin such as feeling faint and dizzy.
  • Keep your regular appointments with your doctor so your progress can be monitored.
  • If you are a smoker, stopping smoking may significantly improve your symptoms. Ask your doctor for advice on quitting.
  • If you are having an operation or dental treatment, tell the person carrying out the treatment that you are taking prazosin. This is because your blood pressure may drop suddenly if you have an anaesthetic. If you are having cataract surgery, it is particularly important that you tell your surgeon you are on prazosin. This is because an eye problem known as 'floppy iris syndrome' has developed in some people and your doctor may advise you to stop taking prazosin for a short while.

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. These usually improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following side-effects continue or become troublesome.

Common prazosin side-effects - these affect less than 1 in 10 people who take this medicineWhat can I do if I experience this?
Feeling tired, dizzy, faint, or weak, blurred visionIf any of these happen, do not drive or use tools or machines. Do not drink alcohol
Feeling light-headed when getting up from a lying or sitting positionGetting up more slowly may help. If you begin to feel dizzy, lie down so that you do not faint, then sit for a few moments to prevent the dizziness returning
Constipation or diarrhoea, feeling sickEat a well-balanced diet and avoid spicy foods. Drink plenty of water
HeadacheAsk your pharmacist to recommend a suitable painkiller
Feeling depressed or nervous, palpitations, dry mouth, rash, shortness of breath, feeling the need to pass urine more frequentlyIf any of these become troublesome, speak with your doctor

If you experience any other symptoms which you think may be due to this medicine, discuss them with your doctor or pharmacist.

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.
  • If you buy any medicines, check with a pharmacist that they are safe to take with your other medicines.
  • Never take more than the prescribed dose. If you suspect that someone has taken an overdose of this medicine, go to the accident and emergency department of your local hospital at once. Take the container with you, even if it is empty.
  • This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.
  • Never keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.
  • If you have any questions about this medicine ask your pharmacist.

Further reading & references

  • British National Formulary; 63rd Edition (Mar 2012) British Medical Association and Royal Pharmaceutical Society of Great Britain, London
  • Manufacturer's PIL, Hypovase® Tablets; Manufacturer's PIL, Hypovase® Tablets, Pfizer Limited, The electronic Medicines Compendium. Dated September 2009.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Helen Allen
Current Version:
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
3229 (v23)
Last Checked:
Next Review:
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