Warfarin is a common drug taken by thousands of Australians every day. Warfarin helps to lower the risk of the blood clots forming.
There are a few things about warfarin that make it different from most other medicines.
- the need to have regular blood tests to monitor the effect of warfarin
- the need for frequent dose changes
- an increased risk of bleeding.
What does warfarin do?
Warfarin is an anticoagulant. You might have heard it called a blood thinner.
Warfarin does not actually thin the blood; it lowers the risk of blood clots forming by increasing the time it takes for blood to clot.
Blood clots can cause stroke, heart attack, or other serious conditions such as blood clots in the legs or lungs.
Why take warfarin?
Common conditions treated with warfarin include:
- pulmonary embolism (PE) – blood clots in the lungs.
- deep vein thrombosis (DVT) – blood clots in veins in the legs.
- atrial fibrillation (AF) – a rapid heartbeat increasing the risk of blood clots and stroke.
- artificial heart valves – mechanical valves increasing the risk of blood clots and stroke.
Warfarin may also be used to reduce the risk of blood clot related complications after some types of heart attacks or surgery.
If your condition is not on this list or you are not sure, ask your doctor why you need warfarin.
The length of time warfarin therapy is needed depends on the condition being treated and your individual risk factors. Your doctor should discuss this with you.
What are the different types of tablets?
There are 2 two brands of warfarin available; Marevan and Coumadin.
Marevan and Coumadin are not the same.
Always use the same brand unless your doctor specifically tells you to change.
They come in different tablet strengths and colours. There are several ways to make up a correct dose using these tablets – your doctor will do this for you.
Check your tablets when you collect your prescription to make sure you have the right brand and strength, especially if you are getting a prescription filled outside of Western Australia.
Marevan® brand warfarin tablets:
1 mg tablet (brown)
3 mg tablet (blue)
5 mg tablet (pink).
Coumadin® brand warfarin tablets:
1 mg tablet (light tan)
2 mg tablet (lavender)
5 mg tablet (green).
Do I need to have blood tests?
Blood tests are needed to monitor the effect of warfarin.
Why do I need blood tests?
The measure of how long your blood takes to clot is called the International Normalised Ratio (INR). In a person not taking an anticoagulant the INR is about 1. The higher the INR, the longer it takes blood to clot.
The INR needed depends on the condition being treated. The doctor will try to keep the INR within a specific range. This is called the target INR range.
For most patients, the target INR range is 2 to 3. An INR in the correct range will prevent blood clots and reduce the risk of bleeding.
If the INR is too high there is an increased risk of bleeding. If the INR is too low there is an increased risk of blood clots forming.
How often do I need blood tests?
Initially, you will need a blood test daily or every second day until the right dose for you is identified. It takes about 1 week to find the right dose for most people.
Once this happens you will usually only need a blood test every few weeks. The frequency of blood tests is different for everybody.
After your blood test your dose of warfarin may be changed to keep the INR within the target range for your condition.
Are there any side effects from taking warfarin?
Bleeding is the most common side effect of warfarin.
Your chances of bleeding increases as your INR rises. Keeping your INR within your target range will reduce this risk.
How can I get started taking warfarin?
The dose of warfarin is changed based on your blood test results.
What dose of warfarin do I need?
The dose of warfarin needed is different for every patient. Your doctor will start with 1 dose and then make changes based on your INR blood test result.
The dosage is often changed in the first few days.
When does warfarin start working?
Warfarin takes 4 to 5 days to reach the target INR. During this time, you may be given injections of another anticoagulant that works immediately. These injections will stop when your target INR is reached.
If you need these injections, make sure you know:
- what time to give yourself the injection
- how to give yourself the injection or know who is going to give the injection
- how many doses you may need.
Your treatment plan
You need to know:
- why you are taking warfarin
- your target INR
- the dose you need to take
- the date of your next blood test, and how to get the blood test.
Fill out your treatment plan on the warfarin treatment card (PDF 260KB).
Where to get help
- See your doctor.
- Visit a GP after hours.
- Ring healthdirect Australia on 1800 022 222.
Medicines information line
- Free independent advice from medical specialists
- Phone 1300 633 424 (local call rates from land line only)
- Read more about the Medicines Line (external site)
Poisons information line
- Immediate specialist advice for overdoses or poisoning
- Phone 13 11 26 (local call rates from land line only)
Adverse Medicines Events Line
National Prescribing Service (NPS) MedicineWise
- Warfarin lowers the risk of blood clots forming in your body.
- Warfarin medication brands Marevan® and Coumadin® are not the same.
- Regular blood tests are needed to monitor the effect of warfarin.
Living with warfarin (PDF 1.34MB)
The WA Medication Safety Group (WAMSG) Anticoagulant Working Group prepared the original version of this book. WAMSG acknowledges:
- Neil Keen, who while chairman of the Working Group and Senior Clinical Pharmacist at Sir Charles Gairdner Hospital prepared the initial draft.
- The initial work carried out Sir Charles Gairdner Hospital through the National Medication Safety Breakthrough Collaborative.
- Nancy Pierce and the WAMSG Community Consultative Group for their valuable input.
- Public Affairs and Marketing, Office of the Director General, Department of Health for their assistance in the preparing the final version.
WAMSG Anticoagulant Working Group (2007) membership
- Dr Ben Carnley (Chairman), Consultant Haematologist, Royal Perth Hospital
- Clinical A/Prof. Ross Baker, Consultant Haematologist, Royal Perth Hospital/University of WA
- Dr Graham Cullingford, Consultant General Surgeon, Fremantle Hospital and Health Service
- Dr James Williamson, Consultant Physician, Sir Charles Gairdner Hospital
- Ann Berwick, Clinical Pharmacist, Royal Perth Hospital
- Tandy-Sue Copeland, Senior Clinical Pharmacist, Fremantle Hospital and Health Service
- Beverley Jones, Senior Pharmacist, Sir Charles Gairdner Hospital
- Stephen Lim, Chief Pharmacist, Armadale Health Service
- David McKnight, Deputy Chief Pharmacist, St John of God Hospital Subiaco
- Susan Cantwell, Clinical Nurse Manager, Fremantle Hospital and Health Service
- Karen Flounders, Thrombosis Nurse Consultant, Royal Perth Hospital
- Dr Margherita Veroni, Project Coordinator, WA Medication Safety Group.
Western Australian Therapeutic Advisory Group | The WA Medication Safety Group
This publication is provided for education and information purposes only. It is not a substitute for professional medical care. Information about a therapy, service, product or treatment does not imply endorsement and is not intended to replace advice from your healthcare professional. Readers should note that over time currency and completeness of the information may change. All users should seek advice from a qualified healthcare professional for a diagnosis and answers to their medical questions.