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.

• 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.

Video: How does warfarin work?

The following animation describes how warfarin works in your body.

(British Heart Foundation, UK, 2018)

In Aotearoa 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're 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.

Image credit: NZ Formulary

The dose of warfarin will be different for different people. It depends on why you're taking it 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.

Take your tablets (as one dose), once a day, at the same time each day. It's 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 medicine organiser or one of the resources in the 'More information' section below.

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. Don't 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 – get a new prescription when you're running low. 

What you eat and drink can affect warfarin. Because warfarin works by interfering with the production of vitamin K, if you have too much vitamin K in your diet it can make warfarin less effective. 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.

If you're ill, especially if you're 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
• not feeling well.

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

Some herbal or 'natural' products (eg, 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're already taking any of these when you're prescribed warfarin, tell your doctor or pharmacist.

It's important to let all your healthcare providers know that you're taking warfarin. This includes your dentist, pharmacist, podiatrist and nurse. You may need to stop taking your warfarin for several days before having surgery or medical tests.

If you're 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're 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's a lot to remember about warfarin. Ask your pharmacist or doctor if there's anything you're unsure of. Use one of the handouts or patient guides in the 'More information' section below.

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're bothering you.

Increased risk of bleeding

Taking warfarin means that your 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're 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.

Read more about medicines and side effects and reporting a reaction that you think might be a side effect.
Find out what to do if you think a child or someone else has taken a medicine that's not for them.