Warfarin, the blood-thinner that’s still used as a rat killer

Warfarin is a blood thinner that has been used for more than 60 years to prevent the formation of clots

Warfarin is a blood thinner that has been used for more than 60 years to prevent the formation of clots and strokes. It is most commonly prescribed for people with atrial fibrillation, deep vein thrombosis, pulmonary embolism and prosthetic heart valves.

Atrial fibrillation – an abnormal heart beat or arrhythmia – affects around 1-2% of the population and is a strong independent risk factor for stroke, a condition where a blood clot blocks the flow of blood to the brain.

Warfarin is an effective therapy as it can reduce the annual risk of stroke by two-thirds, from 4.2% to 1.4%. Patients prescribed warfarin for atrial fibrillation will most likely be on the medication for the rest of their lives.



Warfarin is derived from coumarin, a sweet-smelling anticoagulant (blood-clotting) chemical found naturally in sweet clover and many other plants.

In 1954, warfarin was approved for clinical use and has remained a popular anticoagulant ever since. Today warfarin is one of the most widely prescribed oral anticoagulant drugs with around 1-2% of adults in the developed world prescribed the medication.

Prior to its clinical application in the early 1950s, warfarin was used as a pesticide to kill rats and mice. It is still used for this purpose today.

Despite its evolution, patients are all too aware of and concerned about warfarin’s origins.

How it’s used

In Australia, warfarin is only available in two commercial preparations: Coumadin® and Marevan®.

These are not interchangeable, as the tablets are provided in different strengths and colours. Doctors recommend patients use the same brand of warfarin to reduce any confusion that could occur due the different coloured tablets and their respective doses.

Warfarin is administered once daily and must be taken at around the same time each day in order to eliminate fluctuations of drug concentration in the body.

The dose of warfarin differs with each patient, as their characteristics vary. While some patients require no more than 0.5mg of warfarin per day, others may need 30mg or more per day for effective anticoagulation. The average dose is 4.5mg a day.

Serious problems can occur when blood is too thin or too thick. To prevent this, doctors routinely monitor patients on warfarin via frequent blood tests (at least monthly) and adjust their dose according to the action required.

How much it costs

Warfarin is covered by the Pharmaceutical Benefits Schedule, which means the government subsidises the cost. Consumers pay A$9-12 (or around A$5.20 for concession card holders). In 2001, the cost of subsidising warfarin totalled A$8.3 million. It is now likely to now be much higher.

The blood tests to monitor the use of warfarin (called the International Normalised Ratio or INR) cost the government A$22.52 each, which amounts to more than A$100 million a year.

How it works

Blood clots are formed in the body through a complex process that uses substances called clotting factors. In order to make these clotting factors, the body needs vitamin K, which you can get from many foods including leafy green vegetables.

Warfarin reduces your body’s ability to use vitamin K to make these clotting factors. This results in Vitamin K and warfarin working against each other. Vitamin K is therefore a good, easy antidote for patients whose blood is too thin.

Balancing the benefits versus harms

Warfarin is renowned for its rate of complications and is a common cause of illness and death. Studies based on medical record reviews show that around 11% of drug complications in hospitalised patients result from anticoagulant therapy. Errors due to anticoagulants are more likely to cause permanent disability (32%) than other mistakes.

Warfarin’s main side effect is that it increases the risk of bleeding events, especially among those aged over 65. As a result, you may bruise more readily or experience nosebleeds. It also means that if you cut or injure yourself, you may bleed more than usual.

The rate of minor bleeding among patients with atrial fibrillation is as high as 16% per year.

More serious bleeding events such as a hemorrhagic stroke, where a blood vessel bursts or weakens and bleeds into the brain, are life-threatening. The annual risk of major bleeding (in the brain or bleeding requiring transfusion or hospitalisation) has ranged between 1.2 to 7.0 episodes per 100 patients.

Identifying factors for bleeding risk is key to preventing complications with warfarin. This includes demographic characteristics but also clinical factors such as being aware of warfarin’s interaction with other medications you are taking, particularly antibiotics.

Behavioural and lifestyle factors can also play a role, as you need to monitor your diet for vitamin K and alcohol intake. Psychological and social factors, such as low mood and poor warfarin education – together with how your warfarin is monitored, managed and administered – can increase your risk.

This makes warfarin a very complex medication. On one hand, warfarin usage in Australia has been steadily increasing at a rate of 9% per year due to its effectiveness in preventing a stroke.

On the other, these known risk factors can influence warfarin stability and have resulted in under-use of warfarin in those most at risk of stroke, the elderly.

Warfarin alternatives

In recent years a number of novel oral anticoagulants (NOACs) have been developed – dabigatran, rivaroxaban and apixaban – which promise to be as or more effective in preventing strokes while also being safer and easier to administer because they don’t require regular blood tests.

But these anticoagulants come with their own challenges. There is no quick and easy antidote and there are concerns about their management and effectiveness within a complex and ageing population in long-term therapy.

In the light of this choice, doctors will need to select the appropriate blood-thinning therapy not only based on the individual drug but, more importantly, on the patient’s suitability and the services and supports available to the patient after treatment is initiated.

The ConversationDo you take Warfarin, or some other blood thinner?

Basia Diug, Senior Lecturer & Deputy- Head of the Medical Education Research Quality Unit, School of Public Health and Preventive Medicine, Monash University

This article was originally published on The Conversation. Read the original article.

  1. Thank you for your article. For more than 20 years, I have been on Warfarin, Coudamin. Realising, of course, its origin of Rat bait’ . However, I am ever so grateful for having been blessed and given the chance to experience life as I have. Yes, I have AF. Mitral stenosis, replaced by a mechanical mitral value after many years of different medication and procedures. I had a PaeMaker inserted around 20 years ago, the generator of which has been replaced after 12 years, experienced a couple of TIAs and an ASD repair and a ring around my tricuspid to limit the leakage, which continues to do so. I had open heart surgery just over 10 years ago. I do not see taking it as an issue or the blood tests to determine the INR regularly. Actually, I believe if that is all I have to do to keep me functioning, I have very little to worry about. This drug has kept me alive through many major surgeries and proceduresalong with Clexane and Heparin. Ot just all becomes a process which one just goes with the flow. Over the years, my INR level has become limiting (3.00 -3.5) and thus requiring some more frequent blood tests, all I which I just go with the flow. At this stage of my life, I would never change as it becomes a ‘comfort zone’ to know you are being monitored and actually feel safe and this this I am most thankful. Warfarin has been a blessing to me, having experienced a partial Whipples die to a serous anaenoma in the neck of my pancreas which was beginning to grow over my bile duct. During the surgery, the portal vein was nicked resulting in returning to surgery the next day. I had a fall, tripping on a mat and fell against a door resulting in sliced the whole side of my face open and again, nicked the temporal artery, resulting in emergency surgery with the constant loss od flowing blood from temporal artery. On having a biopsy on my lung u der scan, they accidentally nicked a vein resulting in a Haemothorax and collapsed lung. After hospital stay and draining litres of blood from my lung I was able to go home. All these happened within successive years. I am blessed to be able to continue my life enjoying family and friends and active within my community. Yes, there are the. Look tests, the watching of foods and insuring creams do not cntain vitamin E etc, but to me, these ar e most minor as to the value and guality of life. I am ever so blessed to be able to do so and I thank Warfarin for assisting and allowing me with the grace of God to live life to the full.

    • Jan Cutler  

      Just one question you said watching foods creams containing vit E.. Should that not be vit K ? Also dont forget drinks like cranberry .. fruit like limes ted oranges and grapefruit all interact.. Except for the Vit E and not K I think you are on the right track.. all the best..

  2. Jan Watkins  

    I too am on Marevan, and have been for about 16-17 years. Last year I went to a so called Doctor, at a prominant clinic, I told her I wanted to get my own INR machine as I was going to be travelling outback Australia and not always able to get to pathology to have test done. She in her wisdom took me off marevan and put me on riveroxiban, 3 days later I’m in hospital with clots in my lung and outer wall of heart. Airlifted to Melbourne and there for a week. So now I’m back on marevan and have my own Coagucheck machine.

  3. valerie anne milne  

    ,I have my own Coagucheck machine, if nothing else it gives me a great sense of security and it wasn’t as expensive as everyone kept telling me!. Great for travelling with email contact with my Doctor to alter dosage if necessary as in some places it is hard to keep intake of ‘greens’ stable as you cant get them!

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