Published: 1 December 2022


Warfarin: eat, drink and be wary

Prescriber Update 43(4): 60–61
December 2022

Key messages

  • Warfarin interacts with many medicines, herbal products, dietary supplements and foods.
  • Advise patients on warfarin to avoid herbal products and dietary supplements. Patients should also discuss any major dietary changes with their healthcare professional.
  • More frequent international normalised ratio (INR) monitoring may be required for patients who take herbal products, dietary supplements or certain foods.

The Centre for Adverse Reactions Monitoring (CARM) recently received a case report (CARM ID: 143662) of a patient taking warfarin who experienced spikes in their INR readings. There is a risk of severe bleeding in patients with high INR.1 The patient was taking a turmeric tonic in addition to having a diet high in turmeric.

Medsafe previously published a safety communication warning that turmeric/curcumin-containing products can interact with warfarin.2 However, CARM continues to receive reports of warfarin interactions.

Advice for prescribers and their patients

The Coumadin and Marevan data sheets include information for prescribers on interactions between warfarin and other medicines, herbal products, dietary supplements and food. The interaction checker on the New Zealand Formulary website can also be used to check for known and theoretical interactions.

Patients may not be aware of warfarin’s many interactions, particularly with herbal products, dietary supplements and food. The Coumadin and Marevan consumer medicine information (CMI) leaflets have information for patients about these potential interactions. Health Navigator also has information for patients about warfarin and diet.

Avoid herbal products and dietary supplements3,4

Patients taking warfarin must not use herbal products containing St John’s Wort (Hypericum perforatum). St John’s Wort can reduce the clinical effects of warfarin.

Many other herbal products and dietary supplements have a theoretical effect on warfarin, although most of these interactions are not proven. Advise patients that they should generally avoid taking these products while taking warfarin, and to inform their doctor and/or pharmacist if they are taking any. More frequent INR monitoring is advisable if patients are taking herbal products or dietary supplements.

Avoid major changes to diet3,4

In addition to herbal products and dietary supplements, certain foods such as liver, broccoli, Brussels sprouts and green leafy vegetables can interact with warfarin. These foods are all high in vitamin K, which has anticoagulant properties. A sudden change in diet can affect anticoagulant control. Inform patients to seek medical advice before making major changes to their diet.

Cranberry and grapefruit juice3,4

Patients should avoid cranberry products due to a possible interaction with warfarin. Consider more frequent INR monitoring for any patient taking warfarin and regular cranberry juice. Grapefruit juice may also cause a modest rise in INR in some patients.

New Zealand case reports

As of 30 September 2022, CARM had received a total of 236 reports for warfarin interactions. Of these, 33 were for interactions with food, herbal products and dietary supplements, including 3 with turmeric/Curcuma longa.


  1. Tan CSS and Lee SWH. 2021. Warfarin and food, herbal or dietary supplement interactions: A systematic review. British Journal of Clinical Pharmacology 87(2): 352-74. DOI: (accessed 1 August 2022).
  2. 2018. Beware turmeric/curcumin containing products can interact with warfarin 17 April 2018. URL: (accessed 22 July 2022).
  3. GlaxoSmithKline NZ Limited. 2021. Marevan New Zealand data sheet 18 August 2021. URL: (accessed 1 August 2022).
  4. Viatris Ltd. 2022. Coumadin New Zealand data sheet 19 January 2022. URL: (accessed 1 August 2022).
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