Warfarin and your diet

Published: 22 June 2022

If you or your loved one is taking warfarin, follow our advice to help you know what’s safe to eat and drink. Remember to tell your doctor, nurse or pharmacist if you make any big changes to your usual diet.

What is warfarin?

Warfarin is a type of medication known as an anticoagulant. It may also be referred to as a ‘blood thinner’. Warfarin may also be referred to by the brand names Coumadin® or Marevan^TM.

Anticoagulants make your blood flow through your veins and arteries more easily. This means your blood will be less likely to make a dangerous blood clot. You may be prescribed warfarin to reduce your risk of blood clots forming if you have mechanical heart valves, atrial fibrillation or other conditions that can cause blood clots to form.

While you are taking warfarin you will need regular blood tests. This test is called an INR blood test and it is used to check how long it takes for your blood to clot. This blood test can be done at any Labtests branch. Some GP surgeries and local pharmacies may also be able to do the test.¹ In the beginning, your blood tests will be more frequent (i.e. every 3-4 days) until the INR stabilises and then for most people it will become less frequent (i.e. monthly). It is still very important to have regular INR checks to make sure your blood is in the desired therapeutic range to reduce your risk of blood clots or serious bleeds.

Your doctor will use your INR blood test results to decide on your warfarin dose and will adjust the dose as needed. When you first start warfarin it can take days, or even weeks, to get the correct dose for you. It is normal for your warfarin dose to go up or down. The dose can depend on lots of different things, including what you eat and drink, what other medicines you're taking, and if you become unwell.¹

Foods that affect warfarin

Food containing a lot of vitamin K can affect how warfarin works in your body.^2,3

This is because vitamin K helps blood to clot. Most vitamin K is produced by bacteria in the gut, although there are some foods containing vitamin K.

How much Vitamin K?

The following foods contain varying levels of vitamin K:⁴

Source: New Zealand Foodfiles 2021 Version⁴

Serving sizes based on Eating and Activity Guidelines for New Zealand Adults⁵

Other foods containing Vitamin K include spinach, kale, lettuce, liver, soya beans and egg yolks.

You don’t need to remove any of these foods from your diet.² It’s important to still eat these foods as part of an overall heart-healthy diet because they provide vitamins, minerals and antioxidants to help nourish your body and keep your heart healthy.

What’s most important is that you aim to eat similar amounts of these foods each week.² This will help the level of vitamin K in your blood to stay fairly constant and will make it more likely for your INR level to stay stable.²

Keep your diet consistent

Given what you eat and drink can affect warfarin, it is important to keep your diet consistent.^2,3 This means your dose of warfarin is more likely to stay the same.

Any big changes to what you eat or drink can change how your body responds to warfarin.^2,3 It’s a good idea to speak to your doctor, nurse or pharmacist before making any dramatic changes to your diet, particularly if you plan to eat more or less of the foods that contain vitamin K and which affect warfarin such as green leafy vegetables.³ Your doctor, nurse or pharmacist will advise you on the safest ways to manage this.

Other dietary considerations

Drinking excessive alcohol or consuming large quantities of cranberry juice may cause an interaction with warfarin and increase the risk of bleeding.² However, drinking alcohol and cranberry juice in moderate amounts is likely to be okay.²

Some complementary medicines such as vitamins and supplements can contain substances that interact with warfarin e.g. fish oil, ginkgo or garlic which may mean your warfarin dose will need to be adjusted.^2,3

Tell your doctor, nurse or pharmacist if you’re taking any:

• supplements
• Rongoā rākau (native plant remedies)
• herbal medicines
• vitamins
• alternative therapies.

These can sometimes make your heart medications less effective.

An overall heart-healthy diet

A heart-healthy diet is important for everyone including those who are at high risk of heart disease or living with a heart condition.

To nourish your heart, fill your plate with plenty of whole foods that are close to how they are found in nature such as vegetables, fruit, whole grains, nuts, seeds, legumes and oily fish.

If you are taking warfarin, consider the times of the year or the circumstances in your life which may result in big changes to your diet or eating habits.

These may include:

• During summer when you may be eating more salads and green leafy vegetables.
• Certain times of the year when you may have an abundance of green leafy vegetables in your garden like silver beet, kale and spinach.
• Certain times of the year when vegetables like Brussel sprouts, asparagus or broccoli are in season and more affordable.
• While travelling for long periods of time.
• When sick for long periods of time.
• Following surgery or hospitalisation.
• When altering eating habits to eat more heart-healthy foods or to manage your weight.

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References

1. Best Practice Advocacy Centre New Zealand (BPAC) (2010). Use of INR for monitoring warfarin treatment. 
2. Best Practice Advocacy Centre New Zealand (BPAC) (2017). The safe and effective use of dabigatran and warfarin in primary care. 
3. Queensland Department of Health (2016). Guidelines for warfarin management in the community.
4. New Zealand FOODfiles™ 2021 Version 01 (2021)
5. Ministry of Health (2020). Eating and Activity Guidelines for New Zealand Adults: Updated 2020. Wellington: Ministry of Health.