Binfield Surgery

Terrace Road North, Binfield, Berkshire, RG42 5JG

Telephone 01344 286264

Warfarin – A patient friendly guide

Warfarin is a drug that prolongs the clotting time of blood. It works by inhibiting the production of the proteins that cause blood to clot, known as clotting factors. We need our blood to clot when injured to prevent our bleeding to death.

However, some people have an increased tendency for clots to form within the circulation. This is potentially life threatening; for instance an artery that supplies the brain, eye or foot may become blocked with consequential damage to the part supplied by that artery.

Alternatively, a clot may form in a vein. The main hazard of this is a piece becoming detached and lodging in the small vessels of the lungs. The indications for warfarin treatment are many, but all patients recommended for treatment have in common a tendency to clot within the blood vessels. This may have actually happened in someone -“ for instance a clot in the deep veins of the leg, or a stroke – or may be a potential risk, for example, an irregular heart beat (which leads to turbulent blood flow in the heart), or a metal heart valve replacement.

Some patients with conditions that cannot be rectified are recommended to take warfarin for life. Others that have had a thrombotic event after a clear precipitating factor (usually surgery or trauma) will be advised to stop treatment after a few weeks or months.

Taking warfarin

Warfarin is a long acting drug. It needs only be taken once per day.
Most people take warfarin in the evening. When you start on warfarin this is recommended because the blood test for monitoring (see below) is taken in the morning, and most accurately reflects the effect of a dose taken 36 hours earlier.


We will recommend that you attend for a blood test every few weeks. This is important for monitoring both safety and efficacy of the anticoagulation treatment. You will find that we ask you to come more often at the beginning of treatment, or after a dose change. The blood test measures the INR (International Normalized Ratio) of your blood.

This is a standardized laboratory test that measures the clotting time, under specified conditions, of your blood. The result is expressed as a ratio of this time to the time of clotting of a control sample derived from a pooled sample of normal blood donor plasma. In most people on warfarin we aim for an INR range of between 2 and 3.


The amount of warfarin that you will need to take is not predictable. We have a patient who needs an average of 1mg daily, and one who needs 15mg daily.

Safety and efficacy

In terms of the way you feel when taking warfarin, it has very few side effects. I can honestly state that I have never seen a side effect of warfarin. This is most unusual for any drug used in medicine. Toxicity is however a different matter. Should you be taking too much warfarin, your blood will not clot readily, and you will be at risk of bleeding. Conversely, if you are not taking enough, you remain at risk of clotting (thrombosis) in the blood vessels.

These two considerations explain why monitoring is important, and we would urge you to comply with our recommendations.

Risks vs benefits

It would be idle to tell you that warfarin treatment is entirely safe. We have had two significant episodes of bleeding in our patients in the last eight years who were on the ‘correct’ dose of warfarin as judged by their monitoring blood tests. There have been a further two episodes in people who were overdosed for reasons that were not clear.

However, there is a risk involved in not taking warfarin when indicated. For example, the risk of stroke in someone with atrial fibrillation (a specific type of heart irregularity) is halved when taking warfarin. We will always try to balance these considerations when prescribing warfarin.

Generally speaking, younger fitter patients have fewer problems, but we would not discriminate against people on age alone. Indeed, older people will benefit more from warfarin therapy, at least in atrial fibrillation. We consider that inability to understand instructions would mitigate against prescribing warfarin. We have stopped the treatment in people who declared that they did not wish to attend for monitoring. I consider this entirely reasonable as some people can find it onerous.

Drug interactions

These are potentially important. Aspirin interacts with warfarin by an additive anticoagulant effect. A few people are maintained on aspirin and warfarin, and if the warfarin was started in someone already
on aspirin treatment then this represents less of a problem. However, once you are on warfarin treatment do not take aspirin except on medical advice. The potential for harm is high! This includes some proprietary medicines available without prescription.

Please read the label!

There are two antibiotics in common use that should be avoided if possible, namely erythromycin (and clarithromycin) and metronidazole. Another. although less widely used, is the antituberculous drug rifampicin. You may read on drug information sheets that antiepileptic drugs and cholesterol lowering drugs interact with warfarin.

Most people are on these before warfarin is started, and in this case the monitoring will establish the appropriate warfarin dose. Starting or stopping them when established on warfarin requires close monitoring for a while. Anti-inflammatory painkillers are best avoided, less because of interactions than their potential for causing bleeding from the stomach. Paracetamol and codeine are safe. It is said that certain foods can interact with warfarin. I have heard stories that garlic and onions can do this. The effect is probably small and clinically not significant, but it could explain why we see changes in a person’s warfarin requirement when they have been stable for months.

Alcohol does not have a significant interaction when taken in social quantities! Heavy persistent use is a bad idea anyway, but particularly when you are taking warfarin, as anything that upsets your liver can lead to too much warfarin circulating, as it is here where the drug is broken down.

What can you do?

  • Take your warfarin regularly, in the dose recommended.
  • Don’t run out of tablets!
  • Remind all medical attendants that you take warfarin, particularly if you are being
    prescribed a new medication
  • Attend monitoring when recommended.
  • Ask us for confirmation if your dose has been unexpectedly changed.

We are not above making administrative errors.

In conclusion

Warfarin is a vital part of our arsenal against certain diseases. It is not without hazard, but there is a large body of evidence to suggest that in certain indications in selected individuals the benefits of treatment outweigh the risks.