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Oral anticoagulant therapy with warfarin, phenindione or acenocoumarol

Patient information A-Z

Please take the time to read this information when starting treatment.

What are anticoagulants, and why they are used?

  • An anticoagulant medicine increases the time that your blood takes to clot. These drugs are designed to prevent harmful blood clots from forming in blood vessels. You have been commenced on a vitamin K antagonists (VKA) such as warfarin, phenindione or acenocoumarol (Synthrome®).

How do I take my anticoagulant?

  • You should take your medication once a day at about the same time, preferably on an empty stomach and washed down with a full glass of water.
  • If you miss one dose, or take a different dose to that recommended, please write it in ‘important information’ section of your anticoagulation record sheet or in your oral anticoagulant book, and go back to your normal dose the next day.
  • If you accidentally take more medication than you should, let your GP or outpatient anticoagulant service know so they can assess your risk of bleeding.
  • Contact your GP a week before you are going to run out of medication so you can be given another prescription should you need it.
  • You will be given different strengths of tablet for the drug you are taking in order to make up your dose. Your dose will change depending on how long your blood takes to clot (see below).
  • Your medication should be stored at room temperature, out of reach of children, and not taken after the expiry date on the packet.
  • You should continue taking this medication until your GP or hospital doctor advises you to stop.

Do I need monitoring while taking anticoagulants?

  • Yes, you need to have a blood test called an INR.
  • INR stands for ‘international normalized ratio’. This is a standard test that measures how long your blood takes to clot. Normally blood has an INR of approximately 1.0. The dose that you need to take will depend on your INR test result. If your result is out of the range appropriate to your condition the dose will be increased or decreased accordingly. The dose required to achieve the target INR set varies for each individual.

How does Addenbrookes' anticoagulation service run?

  • The blood sample for INR testing should normally be taken at your GP surgery.
  • Newmarket Road, park and ride site in Cambridge has a drive through blood testing service that can be used without an appointment, but you will need to have with you your yellow anticoagulation book or yellow anticoagulation record sheet. Your yellow anticoagulant dosing sheet (which you will receive via the first class post a few days after each blood test) is folded into three sections separated by perforations:
    • The top section has your name and address.
    • The middle or central section is your record. It has your INR result, the dose of drug to take, and the date of your next INR test in the white box. Please take note of any dose change. Then book your next INR test by the date given.
    • The bottom section is the request form for your next INR blood test.
  • Please answer the questions in this section, the day you have the blood test, and hand to the person taking your blood sample. This form then accompanies your blood sample to the coagulation laboratory at Addenbrooke’s.
  • The date given on your dosing sheet for your next INR test is the last date the test should be performed. If this date is not possible then have a test earlier.
  • Arrange your blood tests Monday to Thursday only.
  • Initially you will have a weekly INR test; however when your INR is within your intended range you will have longer intervals between INR testing.
  • Sometimes the service may telephone you if they want you to change your dose of warfarin more quickly. If this is the case and you are not in they will leave a message on your phone. If you do not want messages to be left please inform the service.
  • Please only telephone the service for your result if you need it urgently.

Are there any side-effects?

Minor side-effects

  • Indigestion/ nausea: try taking your medication after food. If symptoms persist consult your GP.
  • Diarrhoea/ loose stools: Consult your GP.
  • Minor bruising/ minor bleeding: Consult your GP or NHS 111.
  • For women, increased or heavier bleeding during your period: Consult your GP or NHS 111.

Serious side-effects

The most serious side effects of anticoagulant therapy are bleeding or severe bruising.

If you cut yourself, apply firm pressure to the site for at least five minutes using a clean, dry dressing.

You should seek medical advice immediately by attending your A&E (emergency department) if you experience any of the following symptoms:#

  • vomiting or coughing up blood
  • if you suffer a major injury or are unable to stop bleeding
  • if you have a fall or injure yourself while on this treatment especially if you hit your head
  • prolonged nose bleeds (more than 10 minutes)
  • pass blood in your urine (pink or brown in colour)
  • passing blood in your stool (red or black in colour)
  • unexplained dizziness or weakness
  • sudden severe headache or if you suffer a severe blow to the head
  • unusual, severe or spontaneous bruising
  • Severe bleeding during your period
  • If you develop a painful skin rash

What can affect the control of my anticoagulation?

Other medicines and interactions

  • Many drugs interact with warfarin, phenindione or acenocoumarol including prescription drugs, over the counter drugs and alternative remedies. Please always tell the doctor or pharmacist that you are taking these medicines. They will advise you which medicines are safe to take with them.
  • If you start any new medication while you are taking warfarin, phenindione or acenocoumarol we recommend you have a blood test five to seven days after starting the new medication. This way we can ensure that your INR remains within the desired range. If you need a course of antibiotics you should have an INR test three days after starting them.
  • You should not take aspirin unless it has specifically been prescribed by your doctor. It is also advisable to avoid non-steroidal anti-inflammatory drugs like diclofenac, ibuprofen or naproxen. Paracetamol and codeine based painkillers are acceptable.


  • It is important to eat a well balanced diet, keep to a regular diet.
  • Be aware that any major changes in your diet may affect how your body takes up warfarin, phenindione or acenocoumarol.
  • Foods rich in vitamin K may affect your INR result. These foods are important in your diet but eating them in large amounts may lower your INR result. Try to eat the same amount of these foods on a regular basis.
  • Foods high in vitamin K include green leafy vegetables, chick peas, liver, egg yolks, cereals containing wheat bran and oats, mature cheese, blue cheese, avocado and olive oil.
  • Have an INR test if you make any major changes in your diet of more than seven days.


  • We recommend that you do not exceed the national guidelines which are up to 2 units a day for men and women (maximum 14 units a week).These units need to be spread throughout the week. Binge drinking can raise your INR and can put you at greater risk of bleeding.
  • You must not take this medication if you have a condition which puts you at significant risk of major bleeding.

Pregnancy and breast feeding

  • Oral anticoagulants can affect the development of a baby in early pregnancy. Women who are on oral anticoagulants should discuss plans for future pregnancy with the doctor before trying to conceive. Women who think they have become pregnant whist on oral anticoagulants should seek a pregnancy test as soon as possible and if this is positive see their GP urgently. You should avoid breast feeding whilst taking your anticoagulant, discuss this with your doctor.

Who must I tell that I take anticoagulant therapy?

  • Tell any doctor or health care professional prescribing you medicine that you currently take anticoagulant therapy. They will advise you which medicines are safe to take with your anticoagulant.
  • Tell your dentist, or any health worker involved in your medical care.
  • You should carry your alert card with you at all times. If you lose your card please contact the anticoagulant service for a replacement card.

Other questions

  • You may experience heavier periods while you are taking anticoagulant therapy.

For further advice please contact:

Addenbrooke’s Anticoagulation Service
Box 217, Addenbrooke’s Hospital
Hills Road
Tel 01223 217127
Open: Monday to Friday 09:00 to 17:00 hours

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Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151