Warfarin Guidelines

The international normalised ratio (INR) is used to monitor patients receiving warfarin. Warfarin has a narrow therapeutic range, so INR monitoring is necessary to avoid complications from both over-dosage (which increases the risk of bleeding) and under-dosage (which may result in clots).

Oral Anticoagulant therapy

There are two preparations of warfarin (Coumadin and Marevan) in New Zealand. These have different pharmacokinetics therefore, as far as possible, patients should remain on the same product. If a product change has to be made the patient’s INR should be monitored closely. If a patient is known to be allergic to Warfarin then medical management should be discussed with a Haematologist.

Warfarin Dose

It is now recommended that patients should start warfarin on a relatively low dose, either 5mg daily or 3mg daily. In younger patients a higher dose is safe and may achieve control slightly faster.
When a patient begins warfarin treatment they will typically have blood tests every three to five days. As their INR begins to stabilise within a desired therapeutic range the need for blood tests reduces.


Some drugs, foods, herbs and complementary medicines can interact with warfarin


Warfarin should not be used during pregnancy. All pregnant women requiring treatment for thromboembolism should receive subcutaneous LMWH treatment until delivery. After delivery warfarin can be used.


It is safe for women to breast-feed while taking warfarin. A small amount of warfarin is excreted in breast milk but it is not considered to be a risk to the infant. It is recommended that pregnant patients be advised by a haematologist and an obstetrician, as well as their GP.