Phenindione - an anticoagulant

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Phenindione is an anticoagulant. It will be prescribed if you have an unwanted clot in your blood, or if you are at risk of having an unwanted clot.

You will be given a yellow booklet about your anticoagulant treatment; read this carefully.

You will need to have regular blood tests to measure how quickly your blood clots.

What you eat and drink can affect your treatment. Do not change your diet without discussing it with your doctor first.

Type of medicineAn anticoagulant
Used forPrevention and treatment of harmful blood clots
Available asTablets

Phenindione is an anticoagulant medicine, which means that it increases the time it takes for your blood to clot. It works by reducing the effects of vitamin K, which is a vitamin your body uses to make blood-clotting factors.

Phenindione is prescribed to prevent unwanted clots from forming if you have a condition that puts you at risk of this happening, such as atrial fibrillation, or if you have had a heart valve replacement. It is also given to prevent any clots that may have already formed in the blood vessels of your legs, lungs or heart from becoming larger and causing problems. Another anticoagulant, called warfarin, is usually prescribed in preference to phenindione but if warfarin is not suitable for you, you may be prescribed phenindione instead.

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 phenindione it is important that your doctor knows:

  • If you are pregnant, trying for a baby or breast-feeding.
  • If you have any cuts or wounds.
  • If you have a condition that increases your risk of bleeding, such as a stomach ulcer, or if you have had surgery recently, or if you have recently had a stroke.
  • If you have any problems with the way your liver works, or with the way your kidneys work.
  • If you have high blood pressure.
  • If you have been told you have an infection of your heart, called bacterial endocarditis.
  • If you are taking or using any other medicines. This includes any medicines you are taking which are available to buy without a prescription, such as herbal and complementary medicines.
  • If you have ever had an allergic reaction to a medicine.
  • Before you start the treatment, read the manufacturer's printed information leaflet from inside the pack of tablets and any additional information you have been given by your doctor. These will give you more information about phenindione and will provide you with a full list of the side-effects which you may experience from taking it.
  • Take phenindione once a day, exactly as you have been advised by your doctor or anticoagulant clinic. You should aim to take phenindione at the same time each day. This will help keep the levels of the medicine in your blood steady and will also help you to avoid missing any doses. Your doctor will tell you how many tablets to take for each dose. Phenindione tablets are available in three different strengths: 10 mg, 25 mg and 50 mg. Your dose may be made up of more than one strength of tablet. Swallow the tablet(s) with a drink of water.
  • If you forget to take a dose at your usual time, you can take it as soon as you remember if it is still within two or three hours of your usual time. If you do not remember until after this time, do not take the missed dose but do remember to take your next dose when it is due. Never take two doses together to make up for a forgotten dose, and remember to let your doctor know about any missed doses when you next have your blood test.
  • Continue to take phenindione tablets regularly until your doctor tells you to stop. A course of treatment typically lasts from six weeks to three months, although some people may be advised to continue taking the tablets long-term.
  • You will be given a yellow 'Oral Anticoagulant Therapy' booklet; please read this carefully as it gives you important information, such as when you should contact a doctor for advice. You will also be given an 'Anticoagulant Alert Card' which you should carry with you at all times. In case of an emergency, a doctor will need to know that you are taking phenindione.
  • You will need to have regular blood tests to check on how quickly your blood clots. Blood tests may be needed quite often at first but should reduce in frequency quite quickly. The extent to which phenindione is working is measured by the International Normalised Ratio (INR), which is a measure of the ability of your blood to prevent clotting. The amount of phenindione that you need to take will depend upon the result of these blood tests and this is why your dose may change from time to time. The aim is to get the dose of phenindione just right so your blood does not clot as easily as normal, but not so much as to cause bleeding problems.
  • Phenindione may make your urine look pink or orange. This is completely harmless and is nothing to worry about.
  • Changing your diet suddenly can affect your INR, especially if you begin to eat more vegetables and salad than before. You should not begin a weight-reducing diet without discussing it with your doctor first. A major change in diet will mean that you need closer monitoring as your dose may need to be adjusted.
  • Only drink alcohol in small amounts, as this can affect the levels of the medicine in your body. Limit the amount of alcohol that you drink to a maximum of one or two units in any day and never binge drink.
  • If you buy any medicines, check with a pharmacist that they are safe to take with phenindione. For example, you should not take some painkillers (such as aspirin and other anti-inflammatory painkillers) and some vitamin and herbal preparations. If you need to take a painkiller, you may take paracetamol but you should let your doctor know if you need to take it regularly.
  • Because phenindione is used to prevent blood clots from forming, you should try to avoid activities that could cause you to cut or bruise yourself, such as contact sports. Let your doctor know if you have any falls or injuries.
  • You should avoid getting pregnant while you are taking phenindione as it may cause harm to an unborn baby. Make sure you have discussed with your doctor which types of contraception are suitable for you and your partner.
  • If you are due to have any injections, or any medical or dental treatment, please tell the person carrying out the treatment that you are taking an anticoagulant and show them your treatment booklet. It is important that they know you may take longer to stop bleeding. If you are due to have surgery, you may be advised to stop taking phenindione for a few days and be given injections instead.

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the side-effects associated with phenindione - the most common ones are bleeding and bruising. You will find a full list in the manufacturer's information leaflet supplied with your medicine. Speak with your doctor or pharmacist if any of the following continue or become troublesome.

Common phenindione side-effectsWhat can I do if I experience this?
Unusual bleeding or bruising, blood in your urine or stoolsSee your doctor straightaway - your dose will probably need to be reduced
Less common phenindione side-effectsWhat can I do if I experience this?
Skin rash, allergic-type reactions, painful or 'purple' toesSpeak with your doctor as soon as possible
DiarrhoeaDrink plenty of water to replace lost fluids
Feeling or being sickLet your doctor know if this continues
Pink-coloured or orange-coloured urineThis is harmless
Hair loss, changes in the way things tasteIf troublesome, speak with your doctor
Blood disorders, liver problemsYour doctor will check for these

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

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.

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.

Further reading & references

  • Manufacturer's PIL, Phenindione 50 mg tablets; Amdipharm Mercury Company Limited, The electronic Medicines Compendium. Dated March 2012.
  • British National Formulary; 69th Edition (Mar 2015) British Medical Association and Royal Pharmaceutical Society of Great Britain, London

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:
Prof Cathy Jackson
Document ID:
3262 (v24)
Last Checked:
Next Review:
The Information Standard - certified member

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