Cardiovascular / Haematology
Warfarin
Vitamin K antagonist — A long-established anticoagulant that needs regular INR monitoring and dose adjustment.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Warfarin is an oral anticoagulant used where a DOAC is unsuitable — for example with mechanical heart valves, significant mitral stenosis, or in some other specific situations. It is effective but requires regular blood tests to keep it in the right range.
How it works
Warfarin blocks the recycling of vitamin K, which the liver needs to make several clotting factors. Because those factors have to fall before the effect appears, warfarin takes a few days to work and its effect is measured by the INR (international normalised ratio).
In practice
In practice warfarin demands a system: a reliable INR-monitoring pathway, a patient who understands their anticoagulant record, and a low threshold to recheck after any new drug or illness — most antibiotics push the INR up. Reserve it for situations where a DOAC is unsuitable, such as mechanical valves, and make sure the patient knows who adjusts their dose and how often they are tested.
Examples
Practical use
How to take it & use it well
- Take it once a day at the same time, usually in the early evening, so any dose changes after a blood test can be started the same day.
- Your dose is set by a blood test called the INR, and it can change from week to week, so always take the amount you have been told for that day.
- Keep your intake of green leafy vegetables and other vitamin-K-rich foods steady rather than suddenly eating a lot more or less, as this affects your INR.
- If you miss a dose, take it the same day if you remember within a few hours, but never take a double dose. Note the miss and tell your clinic.
- Carry your yellow anticoagulant alert book and show it to any dentist, doctor, pharmacist or surgeon before treatment.
- Report any unusual bleeding, bruising, blood in urine or stools, or a severe headache straight away.
Common uses
- Mechanical heart valves
- Atrial fibrillation where a DOAC is unsuitable
- Recurrent VTE in selected patients
Monitoring
- Regular INR, with dose adjusted to a target range set for the indication
- More frequent INR after any new medicine or illness
- Signs of bleeding
Weighing it up
Advantages & disadvantages
Advantages
- It is a very well established, low-cost blood thinner with decades of evidence behind it.
- It is suitable for some people who cannot use the newer anticoagulants, including those with mechanical heart valves.
- Its effect can be measured precisely with the INR blood test, so the dose can be tailored to you.
- Its effect can be reversed with vitamin K and other treatments if bleeding occurs or before surgery.
- It can be used in some situations of poor kidney function where newer agents are unsuitable.
Disadvantages
- It needs regular INR blood tests, and the dose often has to be adjusted, which is more demanding than newer options.
- It interacts with many medicines and with diet, so changes can shift your INR out of range.
- Too much effect raises bleeding risk and too little leaves you under-protected against clots.
- It takes a few days to reach a stable effect when starting or after dose changes.
- Alcohol, illness and travel can all upset INR control.
Key safety principles
What to watch for
- Narrow therapeutic window — both under- and over-anticoagulation are dangerous.
- Numerous food and drug interactions (including many antibiotics, antifungals and alcohol) shift the INR.
- Avoid in pregnancy (teratogenic).
- Bleeding risk rises sharply above the target INR range.
Key interactions
What to avoid or check alongside
- Many antibiotics can raise the INR and increase bleeding risk, so tell your clinic when you start one.
- Anti-inflammatory painkillers (NSAIDs) and aspirin add to bleeding risk and can irritate the stomach.
- Sudden changes in vitamin-K-rich foods such as spinach, kale and broccoli can lower the INR and reduce protection.
- Alcohol, especially binge drinking, can make the INR unstable and raise bleeding risk.
- St John's wort and some epilepsy medicines can lower its effect, while many other drugs raise it.
- Cranberry juice in large amounts may increase its effect on the INR.
Patient & carer advice
- Keep your INR/anticoagulant record and attend monitoring appointments
- Keep your diet (especially green vegetables) consistent rather than changing it suddenly
- Always check with a pharmacist before starting new medicines, including over-the-counter ones
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Warfarin: frequently asked questions
Why does my dose keep changing?
Warfarin is adjusted to keep your INR blood test in a target range. Food, illness, alcohol and other medicines all affect it, so the dose is fine-tuned over time.
Can I eat green vegetables?
Yes. The key is consistency rather than avoidance. Keep your usual intake steady from week to week so your INR stays stable.
Can I drink alcohol?
An occasional small drink is usually acceptable, but heavy or binge drinking destabilises the INR and raises bleeding risk, so keep it moderate and steady.
Is it safe in pregnancy?
Warfarin can harm a developing baby and is usually avoided in pregnancy. If you are pregnant or planning to be, tell your doctor so alternatives can be arranged.
What should I do before dental treatment?
Tell your dentist you take warfarin and bring your anticoagulant book. You may need a recent INR check, and they will advise on any precautions.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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