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Medicine interactions

Warfarin, diet, alcohol and antibiotics: keeping your INR steady

Warfarin works by reducing the liver's use of vitamin K, and anything that changes vitamin K intake, alcohol load or how warfarin is cleared can move the INR. This guide covers the everyday triggers named by the NHS and what to do when a new antibiotic or dietary change is on the horizon.

Last checked: 2026-10-03

Key facts

At a glance.

  • Mechanism

    The British Heart Foundation explains that blood clots need vitamin K to form and warfarin works by reducing vitamin K production in the liver.

  • Steady is the aim

    The NHS says leafy vegetables such as broccoli and spinach can be eaten but to talk to the warfarin clinic about diet changes, so consistency matters more than avoidance.

  • Items to avoid

    The NHS warfarin page says to avoid cranberry juice and products, grapefruit juice, vitamin K supplements and St John's wort, and not to drink large amounts of alcohol.

  • Antibiotics are named

    The NHS lists antibiotics among the medicine groups that interact with warfarin, so any new course should prompt an INR conversation.

  • INR testing rhythm

    The NHS says tests are every day or every few days at first, then typically every 12 weeks once stable, though some people need them more often.

What the INR is telling you

The INR is a blood test that measures how long blood takes to clot. Your anticoagulant clinic sets a target range for your condition, and the dose of warfarin is adjusted to keep you inside it. Too low and the protection against clots is weaker; too high and the risk of bleeding rises.

Because the dose is tuned to a stable routine, small and sustained changes in diet, alcohol, illness or other medicines can push the INR out of range. The aim is not a perfect diet but a consistent one, and early warning to the clinic when something changes.

Vitamin K foods without fear

The NHS notes that broccoli, spinach and green leafy vegetables can affect warfarin, but that you may eat them and should ask the warfarin clinic for advice. In practice this means eating them in a regular pattern rather than a large salad one week and none the next.

Vitamin K supplements, including those in some multivitamins and nutritional drinks, are a separate issue: the NHS page says to avoid them. Check the label of anything you add to your routine and tell the clinic if you start or stop a product that contains vitamin K.

Alcohol, cranberry and grapefruit

The NHS page for warfarin says not to drink large amounts of alcohol. Heavy or binge drinking can make the INR unstable and increases injury and bleeding risk in its own right. Moderate, regular intake is something to discuss with the clinic rather than guess at.

Cranberry juice and cranberry products and grapefruit juice are listed as things to avoid. These are easy to overlook, because cranberry appears in drinks, supplements and some bladder products people buy for urinary symptoms.

Antibiotics and other short-term medicines

The NHS warfarin page includes antibiotics, anti-inflammatory painkillers, antidepressants, heart medicines, stomach ulcer medicines, antifungals and cholesterol-lowering drugs among those that can interact. Some of these change how fast warfarin is cleared; infections themselves can also move the INR.

Several NHS antibiotic pages list warfarin as a medicine to check. Ciprofloxacin, trimethoprim and amoxicillin are among those that name it. If you are prescribed an antibiotic, say you take warfarin at the consultation, and contact the anticoagulant clinic so they can decide when to recheck the INR.

  • Tell the prescriber you take warfarin before any antibiotic is issued.

  • Contact the anticoagulant clinic the same day a course starts.

  • Expect an earlier INR check than your usual schedule.

  • Ask again when the course finishes, since the effect can reverse.

Painkillers and over-the-counter products

The NHS warfarin page recommends speaking to a doctor before starting any new medicines, which includes things bought in a supermarket. Anti-inflammatory painkillers such as ibuprofen are a particular concern for bleeding, and our dedicated guide on warfarin with NSAIDs explains why.

Herbal products need the same care. St John's wort is named as something to avoid, and there is little reliable information on many others, so a pharmacist check is the safe route.

Recognising trouble

Warning signs of bleeding include blood in urine, black or bloody stools, severe bruising, nosebleeds that do not stop and heavier periods, as the NHS anticoagulant page describes. Contact your GP or NHS 111 promptly for these. Call 999 for severe bleeding, a head injury, or sudden severe headache with confusion.

Carry your anticoagulant alert card and show it to dentists and surgeons. Never stop warfarin without advice from a healthcare professional, because that can increase stroke risk.

Frequently asked

Questions people ask.

Quick, plain-English answers to the questions we hear most.

  • Do I have to stop eating green vegetables on warfarin?

    No. The NHS says you may eat broccoli, spinach and leafy greens but should talk to your warfarin clinic. Eating them consistently is easier to manage than avoiding them.

  • Can I drink alcohol on warfarin?

    The NHS advises against large amounts. Ask your clinic what is reasonable for you, because binge drinking is especially likely to upset INR control.

  • Is cranberry juice really a problem?

    The NHS warfarin page lists cranberry juice and cranberry products among things to avoid, so it is worth taking seriously even though it seems harmless.

  • Will every antibiotic change my INR?

    Not every one, but the NHS lists antibiotics as an interacting group and several antibiotic pages name warfarin. Treat every new course as a reason to alert the clinic.

  • I am being sick and not eating. What do I do?

    Contact the anticoagulant clinic. Reduced food intake and illness can change the INR, and they can advise on testing.

  • Does paracetamol interact with warfarin?

    We did not retrieve a source on this, so ask your pharmacist. The NHS page specifically warns about anti-inflammatory painkillers such as ibuprofen.

  • How often will I need an INR test?

    The NHS says daily or every few days at first and typically every 12 weeks once stable. Changes in medicines or illness usually mean earlier tests.

  • Can I check my INR at home?

    The NHS says some people can self-check at home. Your clinic decides whether that is suitable and how results are shared.

  • Are newer anticoagulants affected by food like warfarin?

    The NHS rivaroxaban page says you can eat and drink as normal. Our DOAC guide covers their separate interaction patterns.

Where this comes from

Sources and checking.

This guide is for information, not medical advice.

Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.

Published 2026-10-03, next review 2027-10-03.

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