CUH NHS 75th birthday logo

Information for patients who require warfarin reversal

Patient information A-Z

Who is the leaflet for?

This leaflet is for patients who have their warfarin reversed with vitamin K (phytomenadione).

You take an anticoagulant medicine, warfarin, which increases the time that your blood takes to clot. The most common reason to take this medication is if you are considered to be at an increased risk of thrombosis, or if you have an irregular heartbeat which may increase your risk of a stroke. Warfarin works by inhibiting the action of vitamin K in the body.

The most serious side effect of anticoagulant therapy is bleeding or bruising. Patients on warfarin need to have a regular blood test known as an international normalised ratio (INR) test.

What is an INR (international normalised ratio)?

The INR measures how long your blood takes to clot, and allows the dose of warfarin to be adjusted to maintain a satisfactory anticoagulant effect. The higher the INR, the longer blood is taking to clot. Depending on the reason for taking warfarin, you will have been advised of the ideal INR range/target for your condition.

What are the causes of a high INR?

Common factors that can cause a high INR include:

  • Taking more warfarin than prescribed.
  • An acute illness or an illness altering how warfarin is taken up by the body.
  • Alcohol consumption greater than national guidelines, or binge drinking.
  • Starting new medications particularly or stopping medications.
  • Starting herbal products or over-the-counter medicines.

Sometimes the cause of a high INR is unknown.

What are the possible signs of a high INR?

  • Bruising more easily.
  • Bleeding more than usual, or unexpectedly; eg a bloodshot eye, blood in the urine, prolonged nose bleeds, heavy menstrual bleeding.

What you need to do if you have a high INR

A warfarin dose reduction or temporary discontinuation of warfarin may be advised. Follow any instructions from your anticoagulant service. It is most likely you will be asked to:

  • Miss one dose of warfarin.
  • Then restart your warfarin at a lower dose.
  • Have a repeat INR blood test within four days.
  • Contact your GP immediately if you have any unusual bruising or bleeding within two days of having vitamin K.

If a patient on warfarin has life threatening bleeding then complete reversal is required immediately in hospital.

If the INR is above 20 then patients with either no bleeding or minor bleeding who are living in the community need to have the INR repeated to confirm the high result. Patients come to Addenbrooke’s to have this repeat test. If the result remains greater than 20 the patient is given between 1mg to 3mg vitamin K intravenously. This usually corrects the INR within eight hours.

If the INR is above 8.0 then patients with either no bleeding or minor bleeding who are living in the community will usually be given 2mg of vitamin K orally as a one off dose. This corrects the INR within 24 hours. This is dispensed at the request of Addenbrooke’s outpatient anticoagulant service by either the community out of hours team, or by the patient’s general practitioner (GP).


Keeling, D & Baglin, T.P. (2011) Guidelines on oral anticoagulation with warfarin - fourth edition. British Journal of Haematology, 10, 277-285.

Further information

If you have any questions, please ask the healthcare team who will be happy to discuss this with you. For further advice please contact:

Addenbrooke’s outpatient anticoagulant service team

Addenbrooke’s Hospital / Box 217

Hills Road



Telephone: 01223 217127

Open: Monday to Friday 0900 to 1700

We are smoke-free

Smoking is not allowed anywhere on the hospital campus. For advice and support in quitting, contact your GP or the free NHS stop smoking helpline on 0800 169 0 169.

Other formats

Help accessing this information in other formats is available. To find out more about the services we provide, please visit our patient information help page (see link below) or telephone 01223 256998.

Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151