Warfarin and how to take it 

Warfarin is a medicine 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.


What is warfarin?

Warfarin is a prescription medicine that helps prevent harmful blood clots forming. It is a type of medicine called an anticoagulant and has been used for many decades.

Warfarin is sometimes referred to as a blood thinner — although this is not how it actually works.

Warfarin increases the time it takes for blood to clot.

People may be prescribed warfarin if they :

  • are at risk of developing blood clots that could cause harm, or
  • already have a blood clot .

For most people, blood clots are useful because they help prevent excessive blood loss if a blood vessel is damaged. However, for some people, blood clots are harmful and even life-threatening. When these harmful blood clots develop they can lead to stroke, heart attack or other health problems.

Some people have health conditions that increase their risk of developing harmful types of blood clots. Taking an anticoagulant medicine like warfarin can help.

There are other kinds of anticoagulant medicines as well. If you have been prescribed a non-vitamin K antagonist oral anticoagulant (NOAC) you can find out more about them here.

Learn more about blood clots

Living with warfarin

If you take warfarin, it is important that you use it safely and correctly.

If you do not take enough you are at risk of blood clots, but if you take too much you are at risk of uncontrolled or serious bleeding.

If you are taking warfarin, follow the tips below to reduce your risk of bleeding and keep the medicine working correctly.

  • Have regular blood tests to check how fast your blood is clotting.
  • Take the exact dose of warfarin your doctor or nurse has prescribed.
  • Continue to take the same brand of warfarin.
  • Make sure your prescriber and pharmacist know all the other medicines you take regularly. A medicines list can be a useful way to keep all the information about your medicines together.
  • Check with your doctor, nurse or pharmacist before you take any new medicines, in case there is a risk of interactions with warfarin. This includes non-prescription and complementary medicines such as herbs and vitamins.
  • Keep 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).
  • Drink no more than 1–2 standard alcoholic drinks per day.
  • Only drink small amounts of cranberry and grapefruit juice, if you have them at all.
  • Tell all your healthcare providers, including your dentist, that you are taking warfarin.
  • Watch for signs of bleeding and report them to your doctor or nurse. Signs include bleeding gums, red or black bowel motions and bruising easily.
  • See your doctor or nurse if you become ill. Having an upset stomach or being sick can sometimes affect your INR (International Normalised Ratio), especially if you are not eating regularly.
  • Wear a MedicAlert ID stating that you are taking warfarin.

Warfarin tests and monitoring

When you are prescribed warfarin, your health professional will organise for you to have regular blood tests that help check how well the warfarin is working. 

The test is called the International Normalised Ratio (INR) and it measures how long your blood takes to clot. The higher your INR, the more time your blood takes to clot (and the higher your risk of bleeding). The lower your INR, the more likely you are to develop a blood clot.

Your target INR

Your INR results need to stay within a certain range. For most people taking warfarin the target INR is between 2.0 and 3.0. The target INR could be slightly higher or lower depending on your health conditions and individual circumstances.

If a blood test result shows an INR above your target range, then your risk of bleeding has increased. If your INR is below your target range, then your risk of developing blood clots has increased.

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 often – eg, every 1 or 2 days for the first week – to work out your correct warfarin dose.

Once you reach your target INR and have a consistent warfarin dose you will need fewer blood tests.

Because many factors affect how warfarin works, it’s important to have your INR tests when they are scheduled. This helps to make sure your INR stays in the target range.

Taking warfarin

Take your warfarin once a day, at the same time, exactly as directed by your prescriber or pharmacist. Try setting 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, keep a record of the dose you are currently taking and the date it was prescribed.

Our NPS MedicineWise warfarin dose tracker is a useful tool to help you record your doses and INR results.

NPS MedicineWise warfarin dose tracker

Date published : 9 December 2020

Keep taking the same brand

Always use the same brand of warfarin that is on your prescription. You should not switch between brands as they are not the same.

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 or nurse.

Know what to do if you forget a dose

If you forget or miss a warfarin dose, what you do next depends on how much time has passed.

  • If it is less than 4 hours since you were scheduled to take your warfarin it is OK to take that missed dose. Then you should go back to your usual timing.
  • If it is more than 4 hours since you were scheduled to take your warfarin, then leave that dose out. Take your next usual dose at your usual time and let your doctor, nurse or pharmacist know that you missed a dose. 

Never take extra amounts of warfarin if you have missed a dose.

Stopping warfarin

If you need to stop taking warfarin, your prescriber will give you instructions on what to do.

You should always talk to your doctor, nurse or pharmacist first before stopping any regular medicine.

Warfarin side effects

The most serious side effect of warfarin is uncontrolled or serious bleeding.

Ask your health professional what signs and symptoms you need to look out for. Some may not be obvious, so it is important to know what they are. 

Make sure you also understand when it is time to contact a health professional about a side effect – and when you need to go immediately to a hospital emergency department.

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.

If you take warfarin and experience bleeding that is unexpected, heavier than usual or takes an unusually long time to stop, contact 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 ill while taking warfarin, talk to your doctor, nurse or pharmacist, even if you do not think it is related to your medicine.

If you have questions about your medicine or need general information about side effects, call Medicines Line on 1300 633 424 (Monday–Friday, 9 am–5 pm)

Warfarin resources and useful links

Fact sheets

Phone services

  • Medicines Line 1300 MEDICINES (1300 633 424) if you have questions about your medicines
  • Healthdirect (1800 022 222) for free immediate health advice from experienced nurses
  • Stroke Foundation’s StrokeLine (1800 787 653) for support and services for someone who has survived a stroke, their family and carers