As a clinical pharmacist of a rehabilitation hospital, I have encountered the use of warfarin frequently for treatment of deep vein thromboembolism (DVT). The majority of patients started on warfarin therapy have had knee or hip surgery and a DVT detected by a doppler scan. This means the expected duration of the treatment is usually three to six months only.

The short duration can pose a problem with compliance and increased risk if the patient is not educated about warfarin appropriately. Starting warfarin therapy can be overwhelming for a person particularly when you go through the blood test requirements and the evidence-based nutrition advice.

Our role is to simplify this information and reassure the person being treated.

Review of the Basics

Warfarin is an anticoagulant that is used to prevent blood clots from forming, or worsening of existing clots. It is available in two different brands, which should not be interchanged without blood monitoring. The warfarin dose should be taken at the same time each day.

Blood Tests

The dose will be determined from the International Normalised Ratio (INR) and adjusted accordingly by the doctor. The INR is a measure of how long it takes for the blood to clot and is determined by a blood test. The target INR is two to three for all indications, except for those who have prosthetic heart valves.

It can take up to five days to achieve a steady INR after starting warfarin. Recommended frequency of INR monitoring is as follows:

  1. Measure INR every two days for the first five days.
  2. If the measurements are consecutively between two and three, the INR monitoring can be increased to weekly.
  3. After this, if there are four consecutive INR measurements, the interval can be increased to twice weekly.
  4. Increase to monthly after another four consecutive measurements.

Side Effects

The most common side effects of warfarin are easy bruising and minor bleeding. If the bruising and bleeding are unexplained, or heavier than usual, they need to be investigated. The doctor should be notified immediately if the patient notices dark red or brown urine and/or dark or black stools, as this is a sign of serious bleeding.

Interactions With Drugs and Food

Many drugs interact with warfarin, which can increase or decrease its efficacy. That is why it is important to always seek advice from a doctor, pharmacist, or nurse about starting or stopping any medicine.

At this stage, I find that many people query their current medicines. It is important to reassure them that they can, and should, continue to take their medicines, as adjusting the dose of warfarin to achieve the target INR takes the effect of these medicines into account.

Herbal medicines, and vitamin and mineral supplements, can also interact with warfarin and advice from a doctor or pharmacist should be sought before taking them. The Warfarin Booklet lists some of the most common.

Finally, there is diet and alcohol. Warfarin inhibits vitamin K dependent clotting factors. Therefore, it is affected by the amount of vitamin K in our diet. Vitamin K is found mostly in green leafy vegetables. In the past, the focus was on reducing vitamin K intake. A certain amount of vitamin K is necessary to maintain the effectiveness of warfarin. The phrase I use often is “consistency is the key in terms of quantity and variety of food” so they do not focus on what they can and can’t eat as much.

Alcohol can affect blood clotting when consumed in large amounts and increase the risk of bleeding. It is best to avoid binge drinking and moderate alcohol intake to one to two standard drinks per day.

Apart from the well-known Warfarin Booklet, I now also refer people to the new book Eating Well With Warfarin.

In conclusion, warfarin dosage varies between patients due to co-morbidities, diet and genetic variation. Providing the key points of warfarin management increases the safe use of warfarin and compliance.


  1. Rossi S, editor. Blood and Electrolytes. In: Australian Medicines Handbook 2012. Adelaide: Australian Medicines Handbook Pty Ltd; 2012.
  2. Chalmers L, Peterson G, Bereznicki L. Warfarin Important Information for Patients.
    St Leonards: Aspen Pharma Pty Ltd. 2011.
  3. Inge K. Eating Well With Warfarin. St Leonards: Aspen Pharma Pty Ltd.; 2012.
  4. National Prescribing Service Limited. Good anticoagulant practice. Medicinewise News 2013; 04 February.

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