Warfarin

Sounds like 'war-far-in'

Warfarin is an anticoagulant. It is used to treat and prevent clots in your blood. Find out how to take it safely and possible side effects.

Checklist if you have started warfarin

Check that your doctor has given you the following:

  • information about your dose of warfarin and duration of treatment
  • a prescription for warfarin with the brand and strength specified
  • your INR target range
  • a blood test form and the date of your next blood test
  • where you can have your blood test
  • a warfarin and INR diary.

What is warfarin?

Warfarin is an anticoagulant. Anticoagulants are often referred to as ‘blood thinners’ but they actually work by interrupting the clot-forming process and increasing the time it takes for blood clots to form. This helps prevent blood clots from forming and stops existing clots from growing bigger.

When is warfarin used?

  • For people who are at risk of stroke. A stroke happens when blood clots form in the heart and travel to the brain (usually due to an irregular heart rhythm known as atrial fibrillation).
  • For people with artificial heart valves, to protect clots developing on the valves. 
  • For the prevention and treatment of deep vein thrombosis, where blood clots form in veins deep within the legs and pelvis. From here, the clots can travel to the lungs and cause a pulmonary embolism.

The following animation describes how warfarin works in your body: 

Source: British Heart Foundation, UK

Different brands of warfarin 

In New Zealand there are 2 different brands of warfarin tablets – Marevan® and Coumadin®. Most people take the Marevan® brand. Marevan® and Coumadin® are slightly different from each other and they are not interchangeable. Therefore, it is important that you stay on the same brand. If your tablets look different from usual, check with your pharmacist.

Both brands come in 3 different strengths and each are a different colour. You may need to take more than one strength of tablet to make up your dose.

How is my dose of warfarin worked out?

The dose of warfarin will be different for different people depending on what you are using it for and your blood test results. You will need to have regular blood tests that measure how long your blood takes to clot. Your doctor will tell you how often you will need a test.

The test is called an INR (international normalised ratio) test. INR tests help your doctor work out what dosage of warfarin is right for you. They will check whether your INR blood test results are within the best range for you (INR range). Read more about warfarin and INR.

The length of time you need to take warfarin depends on what you are using it for and your individual condition. Some people may need to take warfarin for the rest of their lives, while others only need to take it for a few months. Your doctor will talk to you about your situation.

How to take warfarin

Take your tablets (as one dose), once a day, at the same time each day. It is best to take your warfarin in the evening, so you can have your blood test to check your INR in the morning. The test is usually done about 12 hours after taking a dose. You may not need a test every day.

You can take warfarin with or without food.

To help you keep track of when you have taken your tablet, you can use a medication organiser or one of the resources on the right hand side of this page.

What should I do if I miss a dose of warfarin?

If you forget to take a dose, take the missed dose if you remember on the same day. If not, skip the dose and carry on as normal. Do not take 2 doses of warfarin on the same day. Record your missed dose in your anticoagulant booklet and tell your doctor on your next visit. 

Contact your doctor if you miss 2 or more doses in a row. 

Missing doses increases your risk of getting blood clots. Make sure you don’t run out of tablets – refill your prescription early. 

Does food and drink affect warfarin?

What you eat and drink can affect warfarin. As part of a healthy balanced diet, you can continue to eat foods that contain vitamin K in moderate amounts. The most important thing is to keep your regular diet stable. You shouldn't change your eating habits very much as large changes can interfere with warfarin treatment. Alcohol increases the risk of severe bleeding while on warfarin. Alcohol intake should be limited.  

If you make any changes to your regular diet, tell your doctor. Read more about warfarin and diet.

What if I am ill while taking warfarin? 

If you are ill, especially if you are not eating regular meals as you would normally, this may change the effect of warfarin in your body and your INR. Contact your doctor if you experience vomiting (being sick), diarrhoea (runny poos), fever or infection, loss of appetite or you are not feeling well.

Take care when starting new medicines 

Warfarin should not be taken with some medications and herbal supplements, so always check with your doctor or pharmacist before starting any new medicines. This includes over-the-counter anti-inflammatories such as diclofenac (eg, Voltaren Rapid), ibuprofen (eg, Nurofen) and naproxen (eg, Naprogesic).

Some herbal or 'natural' products such as co-enzyme Q10, echinacea, fenugreek, fish oils, dong quai, ginkgo biloba, garlic, ginger, ginseng, glucosamine, kava kava, liquorice and St John’s Wort are known to have an effect on warfarin. Always check with your doctor or pharmacist before taking vitamins, herbal preparations or dietary supplements.

If you are already taking any of these when you are prescribed warfarin, tell your doctor or pharmacist.

Tell your health professionals you are taking warfarin

It is important to let health professionals who are treating you know that you are taking warfarin, such as your dentist, pharmacist, podiatrist and nurse. You may need to stop using this medicine for several days before having surgery or medical tests.

Travelling overseas while taking warfarin

If you are travelling overseas, talk to your doctor about how to manage your warfarin and INR testing. Take enough warfarin with you so you don't run out while you are away. Keep your activity and eating habits as close to your normal routine as you can. If you experience any unusual bleeding or bruising, get medical help.

Ask questions about warfarin
There is a lot to remember about warfarin. Ask your pharmacist or doctor if there is anything you are unsure of. Use one of the handouts or patient guides on this page (see Learn more below).

Side effects of warfarin

Like all medicines warfarin can cause side effects, although not everyone gets them. Common side effects include nausea (feeling sick) and diarrhoea (runny poos). These may go away with time. Tell your doctor if they are bothering you.

Increased risk of bleeding

Taking warfarin means that blood takes longer to form clots, and this can increase your risk of bleeding. The benefits of using anticoagulants usually outweigh the risks of bleeding and your doctor will consider this before starting treatment. You might bleed or bruise more easily while you are taking warfarin.

  • Be careful when shaving, clipping fingernails, brushing and flossing your teeth, or playing sports.
  • Avoid new tattoos and piercings while taking warfarin as these may cause bruising and bleeding. Minor bleeding should usually stop on its own.
  • If you have a fall or hurt your head or body, get medical help immediately, even if you feel okay.

Did you know that you can report a side effect to a medicine to CARM (Centre for Adverse Reactions Monitoring)? Report a side effect to a product


Signs of severe bleeding

If you have any of the following signs of bleeding, contact your doctor immediately or phone Healthline on 0800 611 116 for free 24 hour advice:

  • becoming pale, very weak and tired, or short of breath
  • any bleeding from your gums
  • cuts or nosebleeds that won’t stop (longer than 10 minutes)
  • blood in your stools (poo) – black, tarry stools
  • blood in your urine (pee) – pink, red or brown-coloured urine
  • heavy periods (menstrual bleeding)
  • coughing up blood
  • vomiting up something that looks like coffee grounds.

Learn more

The following links have more information on warfarin:

Warfarin (Māori) New Zealand Formulary, NZ
Starting on warfarin Pharmac, NZ
Reducing harm from high-risk medicines – warfarin Health Quality and Safety Commission, NZ
Taking your warfarin medication BPAC, NZ
Marevan® Medsafe, NZ
Anticoagulants Heart Foundation, NZ

Additional resources for healthcare professionals

Coumadin Medsafe, NZ
Warfarin dispensing in community pharmacies HQSC, NZ

Credits: Written by Sandra Ponen, Pharmacist. Reviewed By: Maya Patel, MPharm PGDipClinPharm, Auckland Last reviewed: 20 Oct 2020