Family doctor

What is Warfarin?

Warfarin is a medicine that prevents blood clotting (coagulating). It is called an anticoagulant or "blood thinner". Another name is Coumadin.

What conditions require anticoagulation?

Conditions which cause excessive clotting of blood and resulting problems:

• Leg and lung clots (DVT & PE)
• Irregular heart rhythm (atrial fibrillation)
• Artificial heart valves
• Blood clots in the heart

Need for regular blood tests

The effect of the medicine on your blood clotting is variable. Regular blood tests (called the 'International Normalised Ratio', INR or 'protime', PT) are necessary to ensure you are taking the correct dose of warfarin to 'thin' your blood. During the early stages of treatment you will need more frequent blood tests, until your warfarin dose is stabilised.

Need for adjusting dose

Following the blood test, you should ring your doctor (usually on the same day) who will advise you on altering your dose of warfarin.

Troubleshooting - erratic blood results

1. Other medications

Many medications interact with warfarin. You should check with your doctor before taking anything new, including 'over the counter' medicines and vitamin supplements (both vitamin K and vitamin C can alter your response to warfarin).

2. Diet

Any sudden changes in your diet, such as crash dieting, may affect warfarin's action.

In particular, the amount of vitamin K in your diet will alter the effect of warfarin, because it is used by your liver to manufacture clotting proteins. For example, during the summer months many people increase their intake of vegetables high in vitamin K (see list below), which increases the amount of warfarin required to maintain adequate blood thinning.

3. Alcohol

Excessive alcohol may increase the effect of anticoagulants, and increase the risk of bleeding. You should not take more than two standard drinks each day.

4. Home monitoring of INR

Talk to your doctor if you would like to investigate this option, where you do your own blood tests at home. This requires you to buy special equipment.

5. Other points

• Take your dose at the same time each day. Anticoagulants may be taken with or without food.
• Make a mark on your calendar when you have taken today's dose.
• Inform other doctors and dentists prior to undergoing any treatment or procedure.
• Discuss with your doctor before undertaking any activity with a high risk of injury.

6. Pregnancy

Anticoagulants should not be taken during pregnancy. Discuss with your doctor before becoming pregnant or breastfeeding.

7. Bleeding problems

Whilst taking an anticoagulant you may be more prone to bleeding, such as when you cut your finger, menstruate or have a nosebleed.

Other signs of possible bleeding that you should report immediately to your doctor are abdominal pain or a change in colour of urine or bowel motions to a dark red or brown.

8. If you miss a dose

If you miss a daily dose, do not take double the next day. Just take your normal daily dose and inform your doctor for further advice.

9. Storage of tablets

Keep your tablets in a safe place where children cannot reach them.