Warfarin belongs to the group of medicines known as anticoagulants (also referred to as a "blood thinner") which work by affecting your body's blood clotting mechanism. Warfarin helps prevent your body from forming harmful blood clots and helps stop any existing clots from getting bigger.

Warfarin is given to people who have some heart problems such as atrial fibrillation or replaced heart valves to prevent stroke, or if a blood clot has formed in the leg or in the lung, to prevent any further blood clots.

Each person's body responds differently to this medication. A blood test called International Normalized Ratio (INR) is used to monitor your warfarin treatment. Your doctor will perform this test regularly and adjust your warfarin dosage accordingly.

Important things to note

• Follow your doctor's instructions about the specific dosage that you need.
• Take warfarin once a day at the same time each day.
• Have regular follow-up and blood test as instructed by your doctor.
• A number of drugs, including western medicines and herbs (e.g. Aspirin, dang gui), can interact with warfarin. This interaction may affect the action of warfarin. A patient taking warfarin should consult the doctor before taking any new medications, including over-the-counter drugs, herbal medicines, supplements, or any other products.
• Warfarin may cause birth defects. A patient who becomes pregnant or plans to become pregnant while on warfarin therapy should notify the doctor immediately.

Common Side-effects

The most common side effect of warfarin is bleeding, which may be serious. It is important to watch out for

• abnormal bleeding, for example, excessive bleeding from a cut, excessive nose bleeding or gum bleeding
• unexplained bruising
• for women, increased bleeding during periods (or any other vaginal bleeding)
• red or black stools
• red or dark brown urine
• abdominal pain or distension
• headache, blurring of vision, weakness, slurring of speech, fainting

It is wise to seek immediate medical advice if you experience any unusual bleeding or symptoms

What to do if you miss a dose

• If you forget to take a dose, take it as soon as you remember the same day.
• Do not take a double dose of warfarin the next day to make up for a missed dose.
• If you do miss a dose, take a note of when this was and inform your doctor in the next visit.

How to store the tablets

• Store in a cool, dry place and away from direct heat and light.
• Keep your tablets out of reach of children.
• Never give it to others, even if their condition appears to be the same as yours.

What precautions should I follow?

• Tell all attending medical doctors, surgeons and dentists that you are taking warfarin.
• Avoid any activity or sport that may result in serious injury.
• Inform your doctor if you suffer from any falls, blows or injuries.
• You should also take special care when brushing your teeth or shaving. Use a soft toothbrush, be gentle when using dental floss; use an electric razor if possible when shaving.
• Avoid taking supplements containing vitamin K, as vitamin K can cause warfarin to be less effective. Many foods, such as green leafy vegetables, contain vitamin K. Patient on warfarin needs to maintain a stable diet and avoid drastic changes in dietary pattern.
• Some food products (e.g. milk) are fortified with vitamin K. Consult your doctor before consuming these products. If you are already consuming these products, consult your doctor before changing your current consumption pattern.
• Avoid drinking alcohol.

Common Q&A

Q: I am worry about the bleeding side effect of warfarin. Can I stop taking it?
A: Before starting warfarin, your doctor has weighed the risks of taking the medicine against the benefit it will contribute. Having regular follow-up and blood test will ensure that the dosage is right for you.

Q: Can I stop taking warfarin or reduce the dosage if I feel well?
A: If the dosage is not enough, you can be at increased risk of developing a clot. Strict adherence to prescription is necessary.

Q: Vegetables contain vitamin K that can affect the effect of warfarin. Do I need to stop taking vegetables while on warfarin?
A: You do not need to stop taking vegetables. Eat a normal and balanced diet to maintain a consistent amount of vitamin K in your body. Avoid drastic changes in dietary habits, such as eating large amounts of green leafy vegetables.

Q: Can I use other medications, such as pain-killer or aspirin, while taking warfarin?
A: Many medications including some pain killers (e.g. non-steroidal anti-inflammatory drugs) interact with warfarin. You should check with your doctor before taking anything new including over-the-counter medicines and vitamin supplements.

Proper use of warfarin helps prevent harmful blood clots from forming within the blood vessels.

Take warfarin exactly as directed by your doctor. Always attend your clinic appointments and have regular blood tests to ensure that the dose of warfarin you are taking is right for you.

If you have any problem, speak to your doctor or pharmacist.

This publication is produced by the Hong Kong Poison Control Network.