Health Centres - Marevan
How does it work?
Marevan tablets contain the active ingredient warfarin sodium, which is a type of medicine called an oral anticoagulant. It is used to stop blood clots forming within the blood vessels. (NB. Warfarin is also available without a brand name, ie as the generic medicine.)
Blood clots normally only form to stop bleeding that has occurred as a result of injury to the tissues. The clotting process is complicated and begins when blood cells called platelets clump together at the site of damage and produce chemicals that activate clotting factors in the blood. Clotting factors are proteins that are produced by the liver. Vitamin K is essential for their production. The activated clotting factors cause a protein called fibrinogen to be converted into another called fibrin. Fibrin binds the platelets together, forming a blood clot. This is the body’s natural way of repairing itself.
Sometimes, however, a blood clot can form abnormally within the blood vessels. This is known as a thrombus and can be dangerous because the clot may detach and travel in the bloodstream, where it becomes known as an embolus. The clot may eventually get lodged in a blood vessel, thereby blocking the blood supply to a vital organ such as the heart, brain or lungs. This is known as a thromboembolism.
Some people have an increased tendency for blood clots to form within the blood vessels. This is usually due to a disturbance in the blood flow within the blood vessels. For example, fatty deposits on the walls of the blood vessels (atherosclerosis) can disrupt the blood flow, giving a tendency for platelets to clump together and start off the clotting process. Slow blood flow in the leg and pelvic veins can also result in clots forming (deep vein thrombosis). These clots can break off and travel to the lungs (pulmonary embolism). A type of fast irregular heartbeat called atrial fibrillation can also disrupt blood flow and may lead to blood clots forming in the heart. Heart valve disease can have the same effect, particularly if artificial heart valves have been fitted. Clots in the heart can detach and travel to the brain, causing a stroke. Warfarin is used to reduce the risk of blood clots forming inside the blood vessels in conditions such as these.
Warfarin works by preventing the vitamin K dependent production of the clotting factors described above. Warfarin prevents the production of these clotting factors by inhibiting the action of vitamin K. Without these clotting factors fibrinogen cannot be converted into fibrin and blood clots are therefore less likely to occur.
The anticoagulant effect of warfarin is measured in terms of the prothombin time, which is the time taken for blood clotting to occur in a sample of blood to which calcium and a substance known as thromboplastin have been added. This time is expressed as the International Normalised Ratio (INR). Your doctor will take regular blood samples and adjust your dose of warfarin as necessary to make your INR fall into the range that has been shown to be effective at preventing blood clots in your particular condition.
What is it used for?
- Preventing abnormal blood clots in conditions with increased risk, eg rheumatic heart disease, atrial fibrillation and after insertion of artificial (prosthetic) heart valves
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Treatment and prevention of blood clots lodged in the veins of the leg
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Treatment and prevention of blood clots that have travelled to the lungs
Warning!
- You should take extra care when participating in physical activities while taking this medicine, as even minor injury may result in bleeding/bruising.
- Because this medicine prevents blood clotting, too much can increase your risk of bleeding. On the other hand, too little will not be effective at preventing clotting. It is therefore important that you have regular blood tests to monitor your blood clotting ability (INR) so that your dose can be altered if necessary. When first starting treatment this test should be daily or on alternate days, and as your INR becomes more stable the tests can be at longer intervals. You will probably need your INR checking after any changes to your warfarin dose, if you become ill suddenly, or if you are prescribed certain other medicines. Talk to your doctor about when your INR will need checking.
- As warfarin works by inhibiting the action of vitamin K, changes to your dietary intake of vitamin K can alter the effect of your warfarin. For this reason, avoid making sudden major changes to your diet, particularly your consumption of green tea, salad and green vegetables (eg broccoli, brussel sprouts, or spinach), which contain large amounts of vitamin K. Large amounts of green vegetables (more than 500g daily) can reduce the effect of warfarin and should be avoided. Changes to your consumption of fats and oils can also alter the effect of warfarin, as vitamin K is a fat soluble vitamin.
- The effects of warfarin can also be altered by other foods, for example soya bean products can reduce the effect of warfarin, and avocados and large amounts of ice cream (over a litre a day) have also been reported to have this effect. Cranberry juice should be avoided, as should large amounts of alcohol, as these may increase the effect of warfarin. Warfarin's effect can also be altered by sudden increases or decreases in your body weight. For these reasons you should avoid making sudden major changes to your diet during treatment with warfarin. For more information talk to your pharmacist.
- Consult your doctor immediately if you experience any bruising, bleeding, dark stools, blood in the urine, vomiting, diarrhoea, fever or acute illness while taking this medicine, so that your INR can be checked.
- You should be issued with an anticoagulant treatment booklet when you start treatment with this medicine. This will contain details of your warfarin dose and INR and will also give advice on your anticoagulant treatment.
- You should tell any health professional treating you, including your dentist, that you are taking this medicine.
Use with caution in
- Elderly people
- Decreased kidney function
- Decreased liver function
- People who have had recent surgery, bleeding or major injury
Not to be used in
- Active bleeding
- People who are more prone to bleeding than normal, eg due to the blood clotting disorder haemophilia
- Severe kidney disease
- Severe liver disease
- Bacterial infection of the heart valves and the lining surrounding the heart (bacterial endocarditis)
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Uncontrolled high blood pressure
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Peptic ulcer
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Pregnancy
This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.
Pregnancy and Breastfeeding
Certain medicines should not be used during pregnancy or breastfeeding. However, other medicines may be safely used in pregnancy or breastfeeding providing the benefits to the mother outweigh the risks to the unborn baby. Always inform your doctor if you are pregnant or planning a pregnancy, before using any medicine.
- This medicine should not be used during pregnancy, especially during the first and third trimesters, as it may be harmful to the developing foetus. Seek medical advice from your doctor.
- Warfarin passes into breast milk in very low amounts, but is not known to have any harmful effects on the nursing infant and can be used safely during breastfeeding. Seek medical advice from your doctor.
Label warnings
- Follow the printed instructions you have been given with this medication.
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