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Prazosin - an alpha-blocker


Your first dose of prazosin may make you feel dizzy or faint, or start sweating. Take your first dose at bedtime and remain lying down until these symptoms have completely passed.

Prazosin can cause dizziness which may affect your ability to drive. Make sure your reactions are normal before you drive and before you use tools or machines.

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About prazosin

Type of medicine

An alpha-blocker

Used for

High blood pressure; heart failure; enlargement of the prostate gland in men; Raynaud's syndrome

Also called


Available as


Prazosin belongs to a group of medicines known as alpha-blockers. It works by blocking the action of certain nerve impulses. This blocking action is useful in a variety of different medical conditions which are listed in the table above, although it is usually used to treat high blood pressure (hypertension).

Prazosin works in people with high blood pressure or with 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.

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

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.

Before taking prazosin

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 knows:

  • If you ever feel dizzy or faint when you stand up, or if you have ever fainted after passing urine.

  • If you need to have cataract eye surgery.

  • If you are pregnant or breastfeeding.

  • If you have any problems with the way your liver works, or any problems with the way your kidneys work.

  • If you have ever had an allergic reaction to a medicine.

  • If you are taking or using any other medicines. This includes any medicines you are taking which are available to buy without a prescription, as well as herbal and complementary medicines.

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How to take prazosin

  • Before you start the treatment, read the manufacturer's printed information leaflet from inside the pack. It will give you more information about prazosin and will provide you with a full list of the side-effects which you may experience from taking it.

  • Your doctor will tell you how many tablets to take each day. This can range from 2-4 tablets a day depending upon the reason why you have been prescribed prazosin. Take the tablets exactly as your doctor tells you to.

  • Swallow the tablet with a drink of water. You can take prazosin either before or after a meal, but you should try to take your doses at the same times of day each day. This will help you to remember to take the tablets regularly.

  • Your first dose of prazosin may make you feel dizzy or faint, so it is important that you take it just before you go to bed. If you feel dizzy or weary, or if you start sweating, remain lying down until these symptoms have completely gone.

  • There are several strengths of prazosin tablet available: 500 micrograms, 1 mg, 2 mg, and 5 mg. When you first start the treatment, your doctor will give you a low dose which may then later be increased. This allows your doctor to make sure that you have the dose that helps your condition and avoids any unwanted symptoms.

  • If you 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.

Getting the most from your treatment

  • Prazosin can cause dizziness, particularly when you first start taking it. This may affect your ability to drive. Make sure your reactions are normal before you drive or do things which would be dangerous if you were not fully alert.

  • Try to keep your regular appointments with your doctor. This is so your doctor can check on your progress. Your doctor is likely to want to take your blood pressure from time to time, particularly when you first start the treatment.

  • You are advised not to drink alcohol while you are on prazosin. Alcohol increases the risk of side-effects from prazosin, such as feeling faint or dizzy.

  • If you are taking prazosin for urinary symptoms, consider reducing or stopping the amount of caffeine you drink (commonly found in tea, coffee and cola). Caffeine can make your symptoms worse, so drinking less of these things may benefit you. Also, if you are a smoker, stopping smoking may significantly improve your symptoms. This is because nicotine irritates the bladder. You can 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.

  • If you buy any medicines check with a pharmacist that they are suitable to take with your other medicines. Some anti-inflammatory painkillers, called non-steroidal anti-inflammatory drugs (NSAIDs), can reduce the blood pressure-lowering effect of prazosin.

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Can prazosin cause problems?

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with prazosin. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects often improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

Common prazosin side-effects (these affect fewer than 1 in 10 people)

What can I do if I experience this?

Feeling tired, dizzy, or faint; blurred vision

Do not drive and do not use tools or machines while affected

Feeling light-headed when getting up from a lying or sitting position

Getting 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 sick (nausea)

Eat a simple but well-balanced diet, and drink plenty of water


Drink plenty of water and ask your pharmacist to recommend a suitable painkiller. If the headaches continue, let your doctor know

Feeling depressed or nervous, the sensation of having a 'thumping heart' (palpitations), blocked nose, dry mouth, rash, feeling short of breath, feeling the need to pass urine more frequently, swollen hands or feet

If any of these become troublesome, speak with your doctor

If you experience any other symptoms which you think may be due to the tablets, speak with your doctor or pharmacist for further advice.

How to store prazosin

  • Keep all medicines out of the reach and sight of children.

  • Store in a cool, dry place, away from direct heat and light.

Important information about all medicines

Important information about all medicines

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. 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.

Do not 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.

Report side effects to a medicine or vaccine

If you experience side effects, you can report them online through the Yellow Card website.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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