A vitamin K antagonist (blood-thinning anticoagulant)
Warfarin
A long-established anticoagulant ("blood thinner") that reduces the blood's tendency to clot, taken long-term with regular INR blood tests to keep it in a safe range.
What is Warfarin?
Warfarin is an anticoagulant that lowers the risk of harmful blood clots — for example in atrial fibrillation, or after a deep-vein thrombosis or pulmonary embolism. It works by reducing vitamin K–dependent clotting factors, so its effect is checked with a regular INR blood test and the amount is adjusted to keep clotting in a safe target range. It interacts with many medicines, foods and alcohol, so consistency and monitoring matter.
Education and reference only. This is a plain-language guide to Warfarin — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Warfarin is one of the oldest and most widely used anticoagulants ("blood thinners") in the UK. It reduces the blood's tendency to form clots, which lowers the risk of a stroke in atrial fibrillation, and treats or prevents clots in the legs (deep-vein thrombosis) and lungs (pulmonary embolism). It is also still the anticoagulant of choice in some specific situations, such as mechanical (metal) heart valves and antiphospholipid syndrome, where the newer DOAC anticoagulants are not suitable. It is taken as a long-term daily tablet, and its effect is measured and fine-tuned with a blood test called the INR.
How it works
Warfarin blocks the recycling of vitamin K, which the liver needs to make several of the proteins (clotting factors II, VII, IX and X) that allow blood to clot. With fewer working clotting factors, the blood clots more slowly, so existing clots are less likely to grow and new ones are less likely to form. Because these factors take time to fall and recover, warfarin's effect builds up over several days and wears off gradually — which is why it is often started alongside a faster-acting anticoagulant, and why the dose is guided by the INR rather than being fixed.
What it treats
Conditions Warfarin is used for
Practical use
How to take Warfarin
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it once a day at the same time, usually in the evening, exactly as your anticoagulant record book says — the amount can change between tests.
- Keep your diet reasonably steady, especially green leafy vegetables (vitamin K); it is consistency, not avoidance, that matters.
- Attend every INR blood test — the amount is adjusted from these results, and missing tests is a common cause of the level drifting out of range.
- If you miss a dose, note it and tell the clinic; do not double up. Keep alcohol modest and steady, as binge drinking can push the INR up.
- Always check with a pharmacist before starting any new medicine, including over-the-counter painkillers and herbal remedies.
Weighing it up
Advantages & disadvantages of Warfarin
Advantages
- Decades of evidence and experience, and low cost.
- Its effect can be measured (INR) and precisely adjusted, and quickly reversed with vitamin K or clotting-factor treatment if bleeding occurs or surgery is needed.
- Remains effective and appropriate where DOACs are not — mechanical heart valves, antiphospholipid syndrome, and severe kidney impairment.
Disadvantages
- Needs regular INR blood tests and dose adjustment, unlike DOACs.
- Interacts with many medicines, foods and alcohol, and has a narrow margin between too little and too much effect.
- Slow to start working and slow to wear off; must be stopped and managed carefully around surgery, and must not be used in pregnancy.
Practical use
Good to know
The single most important thing with warfarin is consistency. The amount needed varies from person to person and is set by regular INR blood tests, often through an anticoagulation clinic, with the aim of keeping the INR in a target range — high enough to prevent clots but not so high that bleeding becomes a risk. Many things shift the INR: other medicines, illness, alcohol and big changes in diet. You do not need to avoid vitamin K–rich foods (green leafy vegetables), but you should keep your intake steady rather than swinging from none to a lot. Everyone on warfarin is given a yellow anticoagulant alert card and record book — carry it, and always tell any doctor, dentist or pharmacist you are taking it.
Who should not take it / use with caution
- People with active significant bleeding, or a very high bleeding risk (for example a recent bleeding stroke or a bleeding stomach ulcer).
- Women who are pregnant or planning pregnancy — warfarin can harm the baby and is normally switched to heparin beforehand; specialist advice is essential.
- Used with great care, and with closer monitoring, in people with liver disease, a history of falls, uncontrolled high blood pressure, or difficulty attending for blood tests.
Monitoring
- INR blood test (regularly, and after any medicine or health change) to keep it in the target range
- Signs of bleeding or bruising
- Review around any surgery, dental work or new illness
Side effects
- The main risk is bleeding — from easy bruising and nosebleeds or bleeding gums to, less commonly, more serious internal bleeding.
- Seek urgent help (999/A&E) for bleeding that will not stop, blood in the urine or stool (or black, tarry stools), coughing or vomiting blood, a severe or sudden headache, or after any significant fall or head injury.
- Rarely, skin reactions or, very rarely, patches of skin damage early in treatment; report a new painful skin rash.
Key interactions
- Warfarin interacts with a very large number of medicines — always have a pharmacist check anything new, including short courses of antibiotics.
- Many antibiotics and antifungals (such as metronidazole, ciprofloxacin, clarithromycin, erythromycin and fluconazole), amiodarone, and some antidepressants can raise the INR and bleeding risk; certain others (such as rifampicin and carbamazepine) lower it and reduce protection.
- Aspirin, other antiplatelets and NSAID painkillers (ibuprofen, naproxen) add to bleeding risk — paracetamol is usually the preferred painkiller. St John's Wort, cranberry juice in large amounts, and heavy or binge alcohol also affect the INR.
Available as: Tablets in different strengths (often different colours), which are combined to reach the amount you need. A liquid is available for people who cannot swallow tablets.
Answers
Warfarin: frequently asked questions
Do I have to avoid green vegetables on warfarin?
No — you do not need to avoid vitamin K–rich foods such as spinach, kale and broccoli. What matters is keeping your intake fairly steady from week to week, because large swings (a sudden crash diet, or suddenly eating a lot more greens) can move your INR out of range. Eat a normal, consistent diet and your clinic can set your dose around it.
What painkiller can I take with warfarin?
Paracetamol is usually the preferred over-the-counter painkiller. Aspirin and anti-inflammatory painkillers such as ibuprofen and naproxen add to the risk of bleeding and are generally avoided unless a doctor has specifically advised them. Always ask your pharmacist before taking anything new.
What is my INR and what should it be?
The INR is a blood test that shows how "thin" your blood is on warfarin. Most people have a target range (commonly around 2 to 3, higher for some heart valves), set by your clinic for your condition. Too low means less protection from clots; too high means more risk of bleeding — which is why the dose is adjusted from your INR rather than being fixed.
Is warfarin or a DOAC better?
For many people with atrial fibrillation or a leg or lung clot, a DOAC (such as apixaban or rivaroxaban) is now first choice because it needs no routine INR testing and has fewer interactions. Warfarin remains preferred in specific situations — mechanical heart valves, antiphospholipid syndrome and severe kidney impairment. The right choice is individual and made with your doctor.
What should I do if I forget a dose?
Do not take a double dose to make up for a missed one. Note the missed dose in your record book and tell your anticoagulation clinic, who will advise. Consistent daily dosing keeps your INR stable, so try to take it at the same time each day.
Authoritative sources
- BNF: Warfarin sodium.
- electronic Medicines Compendium (SmPC): Warfarin (Marevan).
- NICE CKS: Anticoagulation – oral.
Building a medicines information resource?
We create evidence-led, dose-free drug and formulary references for teams.