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Warfarin and how to take it 

Warfarin is a very effective anti-clotting medicine (anticoagulant) used to lower the risk of harmful blood clots. If you are prescribed warfarin, it is important that you know how to use it safely and correctly, to avoid side effects, especially bleeding. 

5 min read

What is warfarin?

Warfarin is one of the oldest and most widely used anticoagulant medicines.

Sometimes it’s called a blood thinner — although this is not how warfarin actually works.

Warfarin works by increasing the time it takes for blood to clot. It is an anti-clotting (antithrombotic) medicine.

Warfarin is mainly used to prevent certain types of blood clots in people who are at risk of blood clots or who already have a blood clot (thrombus). Such clots can cause conditions such as stroke in people with atrial fibrillation, or venous thromboembolism (VTE) in people who’ve had certain surgical procedures. These conditions can be life-threatening if action is not taken to prevent them.

There are other kinds of anticoagulant medicines called non-vitamin K antagonist oral anticoagulants or NOACs. 

Find out more about NOACs and how to take them

Living with warfarin

If you take warfarin, it is important that you use it safely and correctly, because bleeding is one of its main side effects. Living with warfarin means you will need to follow special steps to reduce your risk of bleeding. These include:

  • having regular blood tests to see how much your blood is clotting
  • taking the exact dose of warfarin your doctor has prescribed
  • sticking to the same brand of warfarin
  • checking with your doctor or pharmacist before you use any other medicines, including non-prescription and complementary medicines such as herbs and vitamins, that may cause interactions
  • keeping your diet consistent — especially your intake of foods that are rich in vitamin K (eg, vegetables with dark green leaves and those from the cabbage family)
  • limiting the amount of alcohol you drink to no more than 1–2 standard drinks per day
  • limiting the amount of cranberry juice you drink
  • telling all of your healthcare providers, including your dentist, that you are taking warfarin
  • being aware of the different signs of bleeding — such as bleeding gums, red or black bowel motions, easy bruising — so you can report them to your doctor
  • seeing your doctor if you become unwell – stomach upsets or being sick can sometimes affect your INR, especially if you don’t eat normally
  • wearing a MedicAlert ID stating that you are taking warfarin.

Warfarin tests and monitoring

The test used to monitor the effects of warfarin is called the International Normalised Ratio, or INR. It is a blood test that checks how long it takes for your blood to clot. The higher the INR, the longer it will take blood to clot (and the higher the risk of bleeding). The lower the INR, the more likely you are to develop a blood clot.

Your target INR

Your INR results need to stay within a certain range – target INR  ranges from 2 to 3 for most people taking warfarin. However, some people will need to have a lower or slightly higher target INR.

If your INR is too high, your risk of bleeding increases. And if your INR is too low, your blood is more likely to form a blood clot. Monitoring your INR and keeping it in the target range will help you to avoid side effects.

When you first start taking warfarin you will need to have your INR tested frequently – maybe every 1 or 2 days for the first week – to work out your correct warfarin dose.

Once you’re stabilised on a warfarin dose that achieves your target INR, you won’t need to be tested so often.

Because many factors affect how warfarin works, it’s important to have your INR tests done when they are scheduled. This helps to make sure your INR stays within the target range. Talk to your doctor or pharmacist about any changes to your current medicines or diet – including when you are unwell or travelling – and before taking any new medicines, including those you buy over the counter from the pharmacy, supermarket or health food shop.

Taking warfarin

Take your warfarin once a day, at the same time, exactly as directed by your doctor. You could set yourself an alarm or a reminder on your mobile phone, or use another trigger such as the evening news on TV.

Keep track of your daily dose

Warfarin comes in different tablet strengths; each strength is a different colour. Your doctor may prescribe more than one tablet strength to make up your dose.

Because your dose may occasionally change, it’s a good idea to keep a record of the dose you are currently taking and the date you were prescribed it.

Stick to the same brand

Stick to the same brand of warfarin you are first prescribed - these medicines are not interchangeable.

The tablet colours differ between brands. Check the brand and tablet strength on the medicine label or packaging to ensure you’re taking the right brand and strength as prescribed by your doctor.

Know what to do if you forget a dose

If you forget to take a dose, only take the missed dose if you remember within 4 hours of the time you usually take it.

Don’t take the missed dose if you remember more than 4 hours after the time you normally take it. Take your next dose the next morning or evening at the usual time.

Don’t take a double dose at any time

Keep a note of any missed doses and tell your doctor.

Stopping warfarin

Do not stop taking warfarin or change the dose unless advised to do so by your doctor. Your doctor will tell you how long you need to take warfarin for, and when you should stop taking it.

Warfarin side effects

The most serious side effect of warfarin is bleeding.

To help reduce your risk of bleeding:

  • make sure you follow the instructions for taking warfarin
  • be careful when choosing your activities. Avoid those with a risk of injuries that could cause bleeding, eg, an injury or fall.

Some types of bleeding are more serious than others. Ask your health professional what signs and symptoms you need to look out for, when you should contact your doctor about them and when you need to go to a hospital emergency department. Some signs of bleeding may not be obvious, so it is important to know what these are.

Signs and symptoms of bleeding:

  • bruising
  • bleeding gums
  • red or dark brown urine
  • red or black bowel motions
  • nosebleeds
  • coughing up blood or blood in your spit
  • trouble breathing or swallowing
  • heavier than usual menstrual period
  • bleeding from cuts, wounds and scrapes that takes longer to stop
  • dark or blood-stained vomit
  • severe headache or dizziness
  • unexplained pain, swelling or discomfort.

You may occasionally notice bleeding, such as bruises on your body or bleeding gums when you brush your teeth. If such bleeding concerns you, is heavier than usual or takes an unusually long time to stop, you should speak to your health professional right away.

This is not a complete list of side effects. Occasionally people may experience other side effects with warfarin. If you feel unwell while taking warfarin, even if you do not think it is related to your medicine, you should see your doctor or pharmacist.

Warfarin resources and useful links

5 min read