Anti-clotting drugs

Written by: 
Steve Chaplin, medical writer

A practical guide for people taking anticoagulants such as warfarin.

Anticoagulants are drugs that prevent bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. clotting. You can take them by mouth or inject them. When taken by mouth, they are called oral anticoagulants. This category includes three similar drugs - warfarin, acenocoumarol and phenindione.

If you are taking warfarin or similar oral anticoagulants, key questions and areas of concern include the following:

Getting the dose right

The dose is adjusted by measuring the speed at which your bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. clots, using the International Normalised Ratio (INR).
If you are taking anticoagulants, you will need to have a regular weekly or monthly INR measurement. This can be done for you by your doctor or hospital clinic, a laboratory or a pharmacy - or you can do it yourself (see below).

If you have a higher INR, this means your bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. takes longer to clot.

The INR you should aim for depends on your condition: if you have deep vein thrombosisObstruction of one of the deep veins, often in the calf, by a blood clot. Often abbreviated to DVT., the target INR is 2.5, whereas if you have an artificial heart valveA structure that allows fluid to flow in one direction only, preventing backflow. it is 3.0 or 3.5.1 Usually your doctor will give you a target range to aim for.

Checking your own rate of blood clotting

You can monitor your own INR with a meter. This means that you need to visit the doctor or clinic less often.
Self-monitoring INR can be as good or better than being monitored by a hospital or family doctor - provided that a person wants to do it, completes the necessary training, and the equipment and support are available.2

Do I need to tell people that I am taking oral anticoagulants?

Yes. It's important to make sure that any health professional who treats you or provides medication knows that you are taking an anticoagulantA medication that prevens blood from clotting, or which reduces the likelihood of the blood to clot. - this includes surgeons, nurses, pharmacists, dentists and complementary therapists. Acupuncture is considered safe provided deep needling is avoided.3

Should I wear a medical alert bracelet?

It is advisable at all times either to carry a card or to wear a suitable bracelet so that, in an emergency, people know that you are taking an anticoagulantA medication that prevens blood from clotting, or which reduces the likelihood of the blood to clot..

Other medicines, supplements and your diet

Many over-the-counter and prescribed drugs can alter the effects of warfarin. It is important to make sure that your doctor or pharmacist is aware that you are taking oral anticoagulants, and to check the information leaflet provided with your medicine, before starting, stopping or changing the dose of any other medicine.

Herbs and supplements

Many herbs have the potential to alter the effects of warfarin, though it is not clear how much risk this creates.4-6

St John's Wort in particular greatly increases the effects of warfarin.7 One Canadian study found that co-enzymeA protein that speeds up chemical reactions in the body without being used up itself. Q10 and ginger each increased the risk of bleeding by a factor of 3 - 4, and that the risk of bleeding was increased by taking more than one herbal remedy at a time.8

Tip: For people taking anti-clotting drugs, the safest advice is not to start or stop any other medicines or supplements without first consulting your doctor or a pharmacist.

Food and drink

Foods containing high levels of vitamin K reduce the effects of warfarin.

These include green leafy vegetables, chick peas, liverA large abdominal organ that has many important roles including the production of bile and clotting factors, detoxification, and the metabolism of proteins, carbohydrates and fats., egg yolk, cereals containing wheat bran and oats, mature cheese, blue cheese, avocado and olive oil.

It is healthy to eat these foods but, if you are taking warfarin, it is important not to make sudden major changes in what you eat or how often you eat particular foods without first consulting a health professional.

Cranberry juice increases the effects of warfarin and it is advisable to avoid it altogether if you are taking warfarin.9

Major changes in alcohol consumption, including binge drinking, can also alter the effects of warfarin.9,10

Accidents and bleeding - and what to do about them

You should seek immediate medical attention if you:

  • Have a major injury in an accident
  • Suffer a significant blow to the head
  • Are unable to stop any bleeding.

If you have any of the following symptoms, your bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. may not be clotting as quickly as it should. If you cannot test your own bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. it is important to see your doctor as soon as possible for an urgent INR test:

  • A nosebleed lasting more than 10 minutes
  • Blood in your vomit or sputum
  • Passing bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. in your urine or faeces
  • Severe or spontaneous bruising
  • Unusual headaches
  • For women, heavy or increased bleeding during your period, or any other vaginal bleeding.9

If you cut yourself, apply firm pressure for five minutes using a clean dressing. If the bleeding won't stop, seek medical help.9

What if I become pregnant?

Warfarin, acenocoumarol or phenindione taken during pregnancy can harm the developing foetus. It is therefore important not to take them if you are pregnant or planning to become pregnant.

A woman who is sexually active and is taking oral anticoagulants is therefore advised always to use effective contraceptionA means of preventing pregnancy..

If you think you may have taken one of these drugs while pregnant, it is important to consult a doctor straight away.

Further information

Your doctor or clinic will usually be able to supply you with alert cards and information booklets for people taking oral anticoagulants. These will usually also tell you how to obtain a medical alert bracelet or other product that informs people that you are taking an oral anticoagulantA medication that prevens blood from clotting, or which reduces the likelihood of the blood to clot.. These are recognised by emergency services personnel worldwide.

References: 
  1. Guidelines on Oral Anticoagulation (warfarin): third edition - 2005 update. British Society for Haematology. Br J Haematol 2005;132:277-285. Link (accessed 30 November 2009)
  2. Clinical effectiveness and cost-effectiveness of different models of managing long-term oral anticoagulation therapy: a systematic review and economic modelling. Connock M, Stevens C, Fry-Smith A et al. Health Technol Assess 2007;11(38)
  3. BMAS policy statements in some controversial areas of acupunctureA complementary therapy in which fine sterile needles are inserted into the skin at specific points. practice. Cummings C, Reid F. Acupunct Med 2004;22:134-6
  4. Co-ingestion of herbal medicines and warfarin. Smith L, Ernst E, Ewings P et al. Br J Gen Pract 2004;54:439-41
  5. Potential interactions between alternative therapies and warfarin. Heck AM, DeWitt BA, Lukes AL. Am J Health Syst Pharm 2000;57:1221-7
  6. Herbal and dietary supplement - drug interactions in patients with chronicA disease of long duration generally involving slow changes. illnesses. Gardiner P, Phillips R, Shaughnessy AF. Am Fam Physician 2008;77:73-8
  7. Effect of St John's wort and ginseng on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects. Jiang X, Williams KM, Liauw WS et al. Br J Clin Pharmacol 2004;57:592-9
  8. Risk of warfarin-related bleeding events and supratherapeutic international normalized ratios associated with complementary and alternative medicine: a longitudinal analysis. Shalansky S, Lynd L, Richardson K et al.  Pharmacotherapy 2007;27:1237-47
  9. 'Oral anticoagulantA medication that prevens blood from clotting, or which reduces the likelihood of the blood to clot. therapy. Important information for patients.' National Health Service National Patient Safety Agency. 2008. Link (accessed 30 November 2009)
  10. Joint Formulary Committee. British National Formulary. British Medical Association and Royal Pharmaceutical Society of Great Britain. 57th ed, 2009.