Warfarin and the INR: how it works, targets and counselling points
6 November 2026 · 3 min read · By the PharmacySkills team
Warfarin is the oldest oral anticoagulant still in wide use, and it is one of the medicines most often linked to serious harm. That makes it a favourite with examiners and a medicine every pharmacist needs to understand properly.
How warfarin works
Your liver needs vitamin K to make four clotting factors: II, VII, IX and X. Each time vitamin K does its job, it has to be "recycled" before it can be used again. Warfarin blocks that recycling step, so the liver makes fewer working clotting factors and the blood clots more slowly.
Two things follow from this:
- It is slow to start. Clotting factors already in the blood have to wear off first, which takes several days. When someone needs anticoagulating quickly, for example with a new DVT, a heparin is given alongside warfarin until the INR is in range.
- Vitamin K reverses it. That is why vitamin K is the antidote, and why a sudden change in how much vitamin K someone eats changes their INR.
What warfarin is used for
- Preventing stroke in atrial fibrillation, where a DOAC is not suitable.
- Treating and preventing deep vein thrombosis and pulmonary embolism.
- Mechanical heart valves. Here warfarin is the only oral option, because DOACs are not licensed for them.
The INR and target ranges
Warfarin's effect varies a lot between people and over time, so it is monitored with a blood test called the INR (international normalised ratio). An INR of 1 is normal clotting; an INR of 2.5 means the blood takes about two and a half times as long to clot.
| Indication | Target INR |
|---|---|
| Most indications (AF, DVT, PE) | 2.5 (range 2 to 3) |
| Recurrent DVT or PE while in range; some mechanical valves | 3.5 (range 3 to 4) |
Interactions that matter
Warfarin interacts with a long list of medicines. You do not need to memorise them all, but you should recognise the usual suspects.
These can raise the INR (more bleeding risk): clarithromycin, erythromycin, metronidazole, co-trimoxazole, fluconazole and amiodarone. Regular paracetamol can raise it too.
These can lower the INR (more clotting risk): carbamazepine, phenytoin, rifampicin and St John's wort.
These add to bleeding risk without changing the INR: NSAIDs and aspirin, unless they have been prescribed deliberately.
The practical rule: whenever an interacting medicine is started or stopped, the INR should be checked.
Counselling points
When you counsel someone on warfarin, cover these:
- Take it at the same time each day, usually the evening, so dose changes from the clinic can be made the next day.
- Keep the yellow anticoagulant book and carry an alert card.
- Keep vitamin K intake steady rather than avoiding green vegetables altogether.
- Avoid cranberry and grapefruit juice, and keep alcohol low and steady.
- Use paracetamol for pain rather than ibuprofen or aspirin, and check with a pharmacist before buying any medicine or herbal remedy.
- Report bleeding: nosebleeds that will not stop, unusual bruising, black stools or blood in the urine.
- Warfarin can harm an unborn baby, especially in the first trimester. Anyone who could become pregnant should get advice before trying.
Exam tip
Questions often hide a warfarin interaction inside a busy scenario. A patient on warfarin who is prescribed a course of clarithromycin for a chest infection is a classic. The answer is rarely "never give it". It is usually "give it, and arrange an extra INR check".
For another heavily examined cardiovascular class, read our guide to ACE inhibitors.
Sources: BNF: Warfarin sodium; BNF: Oral anticoagulants, treatment summary; NICE Clinical Knowledge Summaries: Anticoagulation, oral; NPSA: Actions that can make anticoagulant therapy safer.
This article is for revision and education. Always check the current BNF, NICE guidance and your local policies before applying it in practice.