Warfarin

What is this medication for?

Warfarin is an anticoagulant, sometimes also known as a “blood thinner”. It works by blocking the action of Vitamin K in the liver, which is required to form blood clots. 

It prevents harmful blood clots from forming in your heart and blood vessels and getting stuck elsewhere such as the brain, lungs or legs. It does not dissolve existing clots but prevents clots from becoming larger. 

It reduces the risk of events such as a stroke or heart attack, which can happen due to the formation of blood clots.

Warfarin may be prescribed if you have:

• Existing clots in the lungs, veins, arteries or heart 
• Irregular heart rhythm 
• Heart valve replacements 
• Blood disorders that result in increased tendency to form clots
• Stroke 
• Others: to reduce the clotting ability of your blood to prevent further complications.

The duration that you may need to take warfarin for can vary from three months to long term, depending on your condition. You should check with your doctor or pharmacist how long you need to take warfarin for.

Warfarin brand

The most common brand available in Singapore is Marevan®. There are 3 colours and strengths: 1mg (brown), 3mg (blue), 5mg (pink). Do not change the brand of warfarin on your own as they may have slightly different effectiveness.

Importance of INR monitoring while on warfarin

Warfarin dose varies among individuals and the dose you require depends on a blood test known as the International Normalized Ratio (INR).

International Normalized Ratio (INR):

• Measures how long your blood takes to clot.
• Your doctor will set a target range for you.
• If your INR is below target – your blood may be thicker than desired, which means a higher risk of clotting 
• If your INR is above target – your blood may be thinner than desired, which means a higher risk of bleeding
• You will need closer monitoring of the INR blood tests if you are newly started on warfarin or had a recent change in your warfarin dosage.

How should I take or use this medication?

You should take warfarin once a day, with or without food. Take your medication at the same time each day. Do not miss, double or adjust your dose unless directed by your doctor or pharmacist.

What should I do if I forget to take or use this medication?

If you miss a dose,

• Take the missed dose as soon as you remember if it is within 8 hours of your usual time to take warfarin.
• If it is after 8 hours, skip the dose for that day and take your warfarin dose at the usual time the next day.
• DO NOT double your dose unless your doctor or pharmacist tells you to, as this can cause excessive bleeding.
• Record all missed doses and dates. Inform your doctor or pharmacist at your next appointment.

What precautions should I take when taking or using this medication?

• Avoid any contact sports or activities that may cause you to be injured and lead to excessive bleeding. 
• Be extra cautious when handling razors or other sharp objects. 
• See immediate medical attention if you have a fall or have an injury to your head or other parts of your body as serious internal bleeding may occur. 
• Inform other healthcare professionals (doctors, dentists, pharmacists or nurses) whom you are seeing for other condition(s) that you are on warfarin, especially if you:
  • Will be going for any surgical procedures, including dental treatment. Your doctor or dentist will advise you if there is a need to interrupt your therapy, to lower your risk of bleeding.  
  • Require an injection into the muscle as you are at higher risk of bleeding in the muscle.
• Ensure that you always have sufficient warfarin supply especially if you need to travel overseas.
• Discuss with the doctor or pharmacist if you:  
  • Undergo acupuncture or massage as they may result in bruising or bleeding 
  • Are pregnant or intending to get pregnant as warfarin may affect your foetus
     
• Carry your anticoagulation record booklet or other identification stating that you are on warfarin. This will ensure you receive appropriate treatment if you are involved in an accident and taken to hospital.

What are some common side effects of this medication? ​

NIL

What are some rare but serious side-effects that I need to seek medical advice immediately? 

Monitor for signs and symptoms of bleeding¹ while on warfarin as bleeding may occur even when the INR is low.

¹Signs and symptoms of low blood count include feeling very tired and weak, having shortness of breath, loss of appetite, pale or yellowish skin, giddiness and chest pain. 

The symptoms of a drug allergy include one or more of the following:

• Swollen face/eyes/lips/tongue
• Difficulty in breathing
• Itchy skin rashes over your whole body

If you experience any of these symptoms, you should stop your medication and see your healthcare professional immediately.

What food or medication should I avoid when I take or use this medication?

Dietary interactions with warfarin

The effect of warfarin depends on the amount of Vitamin K in your body which is mostly derived from your diet. A change in the Vitamin K intake from your diet will affect the INR result and subsequently the effect of warfarin. Maintain a consistent diet in terms of your daily portions of vitamin K-rich food to ensure consistency in Vitamin K intake.

Some examples of food rich in Vitamin K: 

Inform your doctor or pharmacist if you intend to make big dietary changes e.g. becoming a vegetarian.

Check with your doctor if you are unable to eat for several days, have continuous stomach upset or diarrhoea as this may reduce the amount of Vitamin K and affect warfarin absorption in your body.

Avoid any form of traditional or herbal remedies (e.g. ginseng soup, herbal “Bak Kut Teh”, herbal jelly). 

Avoid taking cranberry or grapefruit in large amounts as these may affect the warfarin level in the body.

Avoid excessive alcohol consumption. It is advisable to take no more than 1 can/glass of beer or wine per day. Avoid strong alcoholic drinks (e.g. whisky, brandy) as this can cause changes to the effect of warfarin.

Medication interactions with warfarin

Many medications can affect the way warfarin works in your body. 

Consult your doctor or pharmacist before you start, stop, or change the dose of your medications. If there are any supplements that you are keen to start or change, consult your doctor or pharmacist as well. You may need to have more frequent monitoring on the effect of warfarin. 

Examples of medications that may affect warfarin: 

• Prescription medications for thyroid disorders, high cholesterol, seizures (fits), stomach ulcers, antibiotics and other blood thinners
• Over-the-counter medications or painkillers
• Nutritional supplements such as multivitamins oe omega-3-fish oil

How should I store this medication?

Store in a cool and dry place, away from direct sunlight. Keep this medication away from children. 

How do I throw away this medication safely? ​

Pack this medication into a black trash bag and seal it tightly before throwing it into the rubbish chute or bin.

What else should I know about this medication? 

Blood clots may still form while you are taking warfarin. The signs and symptoms of the blood clots forming will depend on the site of the blood clot.