Warfarin: an expert's view

Dr Peter MacCallum

Dr Peter MacCallum, whose research career has focused on the management of blood clots in veins and arteries, talks to Judy O'Sullivan.

Patient concerns

Warfarin is a highly effective way of reducing the risk of stroke in patients with atrial fibrillation (AF). However, I do meet patients every week in the hospital who mistakenly refer to warfarin as rat poison.

Warfarin does need careful management to minimise the risk of major bleeding complications, which do occur but thankfully are uncommon. In some instances warfarin cannot be used because the individual concerned has a particularly high bleeding risk.

Many doctors also don’t like to prescribe warfarin as they think their patients won’t be able to cope with the inconvenience of having their blood regularly monitored. Sadly it is often the patients who are most likely to benefit from warfarin who are not offered it.

How warfarin was developed

Warfarin’s purpose is to lengthen the time blood takes to clot and thereby reduce the risk of a potentially fatal blood clot forming.


Scientists at the time recognised the huge potential of this discovery: the anti-clotting properties of dicoumarol could be both beneficial for medical use in humans and harmful in pesticides for rodents. The similarities, however, end there. Rat poison and warfarin have very different aims and contain very different levels of dicoumarol. The compound is much higher in rat poison products than in the warfarin because the purpose of the rat poison is to cause death from bleeding to a small mammal while warfarin’s purpose is to lengthen the time blood takes to clot in a human and thereby reduce the risk of a potentially fatal blood clot forming.

About the expert

Dr Peter MacCallum is Clinical Senior Lecturer in Haematology at Barts and The London School of Medicine and Dentistry, Queen Mary University of London as well as Honorary Consultant Haematologist at Barts and The London NHS Trust.

Benefits of warfarin

As Mrs Paton’s story shows, warfarin is very effective at reducing significantly the risk of stroke in people with AF. In fact research in the late 1980s and early 1990s showed warfarin was considerably better at this than aspirin.

The more risk factors the patient has, the higher the risk of stroke, and greater the need for warfarin.


When assessing an individual with AF’s risk of stroke, we take into account the patient’s age, blood pressure, whether they have diabetes, heart failure or a history of strokes or mini strokes. The more risk factors the patient has, the higher the risk of stroke, and greater the need for warfarin which we know, from research, reduces the absolute risk of stroke associated with AF by 60 to 70 per cent - roughly a two-thirds reduction. So if a person, because of their personal risk factors, has a 12 per cent per year risk of having a stroke, warfarin can reduce that risk to about four per cent.

All drugs have side effects, and warfarin is no exception. Patients can experience serious bleeding. The risk of a bleed as a result of taking warfarin is approximately two per cent per year. In other words the potential benefits of taking warfarin far outweigh the potential risks for most people.  The benefit-to-risk ratio must always be taken into account when prescribing any drug. Drugs are considered safe when potential benefits for a patient outweigh potential risks.

Why regular monitoring is needed

The level of warfarin in the blood varies with time and is affected by a variety of things such as acute infections or other illnesses, levels of alcohol intake, significant changes to diet such as loss of appetite for several days, and interaction with other drugs. This means the dose needs to be adjusted frequently which means regular monitoring.

Most patients adjust to the inconvenience of blood tests because they want to benefit from a reduced risk of stroke.

Most patients adjust to the inconvenience of blood tests because they want to benefit from a reduced risk of stroke


Over the years, processes for monitoring blood and prescribing warfarin have improved hugely. Patients can even do this at home, taking blood from a fingerprint, testing it using small portable devices and phoning or emailing the results to GPs or the local anticoagulation clinic. Some patients are able to manage their own warfarin dosing with minimal involvement of GP or hospital clinic.

Blood samples are monitored using the standardised ‘international normalised ratio’, or INR, and now there’s computer software to help health professionals prescribing warfarin to work out the most appropriate dose according to the INR level.

Warfarin is a well-tested drug

The longer a drug has been around the more we know about it. Warfarin has been in use for 60 years so scientists and doctors have had plenty of opportunity both to research it extensively and observe real-life patients taking it.

Warfarin continues to be taken by millions of people of all ages worldwide


With newer drugs, we have clinical trial data alone rather than years of real-life experience. Clinical trials can paint a slightly false picture of a drug because patients are more closely monitored and are usually healthier and younger than patients taking the drug in real life.

Warfarin continues to be taken by millions of people of all ages worldwide. Every day of every year it reduces the risk of stroke in people like Mrs Paton and that, in my view, has to been viewed as a major benefit.

Read a patient's view on warfarin

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