What is Warfarin?

Warfarin is an anticoagulant drug prescribed to prevent clots forming in the blood or to treat clots that have already appeared.

Anticoagulants may also be referred to as "blood thinners". Although they do not actually thin the blood, they do lengthen the time it takes for the blood to clot. The test to measure this clotting time is called an International Normalised Ratio or INR. This test enables the doctor or nurse specialist to adjust the warfarin dose to suit you.

The nurse specialist, pharmacist or doctor will decide the strength and amount of warfarin that you need, depending on the result of your INR blood test. Different colour tablets equal different strength tablets (please see below).

Different strengths of Warfarin

Warfarin works best if it is taken at the same time each day. We suggest you select a convenient time to take your tablet that is easy for you to remember and stick to it every day. If you forget to take your warfarin on one day, just carry on with the prescribed dose the following day at your normal time. Do not double up on the dose.Some people find it helps to use a calendar and mark off each dose by a line through the date. In this way you are unlikely to forget.

If you take your warfarin as directed and have regular blood tests then you should not have any problems taking warfarin.

Warning signs of incorrect dosage of Warfarin

The following are indicators that the dose of warfarin is or has been too high:
  • Severe bruising
  • Prolonged nose bleeds
  • Bleeding from gums
  • Severe bleeding from a small cut
  • Unusually heavy bleeding during periods
  • Passing black ‘tar-like’ bowel movements that are foul-smelling
  • Passing blood in urine.
If you experience any of the above symptoms you must go to your nearest hospital Emergency department so that your INR blood test can be done immediately. Please remember to take your yellow record book and your most recent clinic appointment letter with you.

Drugs that interact with Warfarin

Some drugs can interact with warfarin and should usually be avoided where possible. If this is not possible you may require more frequent monitoring. You should check first with your GP or the doctor in the anticoagulant clinic.

Always ask your chemist before buying any remedies or medications that they are safe to take with warfarin.

Some common examples of drugs that can increase or reduce the effect of warfarin are:
  • Aspirin
  • Thyroxine
  • Allopurinol
  • Co-proxamol
  • Spironolactone
  • Simvastatin
  • Cimetidine
  • Ibuprofen
  • Tamoxifen
  • Oral contraceptives
  • Some antibiotics
In addition, we advise you not to take any health shop herbal remedies such as St John's Wort, Cod Liver Oil and Vitamin capsules. Please note cranberry juice can also interfere with Warfarin.
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