Too much warfarin (over-anticoagulation): signs, causes and what to do

Warfarin is an anticoagulant, sometimes called a blood thinner. The BHF explains that, although the name suggests otherwise, it does not thin the blood. It makes it harder for the blood to clot by reducing the effect of vitamin K, which clots need in order to form. The NHS lists the reasons it is prescribed: treating and preventing clots in the veins and lungs, and reducing the risk of clots in people with an irregular heartbeat (atrial fibrillation), a mechanical heart valve, a clotting disorder or a previous transient ischaemic attack (TIA).

NHS inform describes warfarin as decreasing the blood’s tendency to clot without stopping it completely, which is why the dose has to be monitored carefully. Over-anticoagulation is the term used when the effect is stronger than intended, so that the blood takes too long to clot. It is not a diagnosis you can feel. It is found on an INR test or because bleeding has started.

The NHS says that while you take warfarin you need regular blood tests that measure how quickly your blood clots, called the international normalised ratio (INR). When you first start, you will need tests every day or every few days. Once you are on a stable dose, you usually only need a test every 12 weeks, though some people need them more often. Some people can check their INR at home.

Your dose may be altered according to your results. A result outside the range your clinic aims for is the usual way over-anticoagulation is discovered, often before any symptoms occur. If you are told your INR is high, your anticoagulation clinic or GP will advise on the next step, which is decided individually and not something to adjust yourself.

Several things are described in the sources as able to alter the effect of warfarin or increase the bleeding risk.

• Taking too much. The NHS says extra doses can be dangerous. Do not take two doses to make up for a missed one.

• Other medicines. The NHS lists NSAIDs such as ibuprofen or aspirin, antibiotics, antidepressants, medicines for heart problems, stomach ulcers or fungal infections, and cholesterol-lowering medicines such as statins as types that may not mix well. NHS inform advises avoiding aspirin and ibuprofen unless prescribed and says paracetamol can be taken within the recommended dose.

• Herbal remedies and supplements. The NHS says not to take St John’s wort or vitamin K supplements, and NHS inform says herbal medicines and supplements can interact.

• Food and drink. The NHS says not to have cranberry products or grapefruit juice and not to drink large amounts of alcohol. NHS inform says binge drinking is dangerous and that heavy drinkers or people with liver disease should not drink alcohol.

• Diet changes. Significant changes in vitamin K intake can interfere with how warfarin works, so speak to your clinic before changing your diet.

• Illness. NHS inform says you are at higher risk of bleeding in the first few weeks and when you are unwell.

The NHS lists symptoms of serious bleeding, and NHS inform lists symptoms that should prompt a call to NHS 111.

If you have taken more than your prescribed dose, the NHS says to get help from NHS 111 straight away, even if you feel well. Have your medicine and your treatment record to hand. Follow the advice you are given rather than skipping or changing doses yourself.

If you have bleeding symptoms, a head injury or have been in an accident, follow the advice above. Show your anticoagulant alert card to the clinician. The NHS says you will be given one and should always carry it, and show it to your doctor or dentist before any medical or dental procedure.

Treatment for a high INR or for bleeding is decided by clinicians and depends on how high the INR is and whether there is bleeding. It can range from dose adjustment and closer testing to hospital treatment. The pages used for this guide do not give specific INR thresholds, so those details should come from your own anticoagulation service.

Taking warfarin at the same time each day, keeping a record of doses and missed doses, and attending every INR test are the main protections. The NHS suggests noting any missed doses and contacting your warfarin clinic if you miss them often. NHS inform advises taking extra care to avoid cuts, such as being careful when shaving, brushing teeth gently and wearing protective clothing when gardening or playing contact sports.

Check with a pharmacist before taking any new medicine or supplement, including over-the-counter products. Tell dentists and surgeons that you take warfarin, because the NHS inform page says the dose may need to be lowered or stopped a few days before an operation or dental work.

Warfarin is not suitable for everyone, and pregnancy is a particular concern. The NHS says to tell your doctor straight away if you are pregnant or planning a pregnancy and not to stop warfarin unless told to.

Warfarin monitoring on the NHS is run by GP practices or anticoagulation clinics, and the NHS says some people can self-test at home. If you see a private doctor or dentist, make sure they know you take warfarin and your most recent INR, because treatments such as new antibiotics can change your result. Emergency bleeding is dealt with by NHS urgent and emergency care regardless of where your prescription came from.