An anticoagulant (blood thinner) · Written by the Lyng Pharmacy team, based on NHS and BNF guidance · Updated August 2026
Also sold asCoumadin
Type of medicineAnticoagulant
Available asTablets, oral suspension
How you get itPrescription only
Warfarin prevents dangerous blood clots, and it demands more of the person taking it than almost any other medicine — regular blood tests, a consistent diet, and real care with other medicines. Newer anticoagulants avoid much of that, but warfarin remains the right choice for some people, including anyone with a mechanical heart valve.
What it is used for
Preventing and treating blood clots — deep vein thrombosis and pulmonary embolism — and reducing stroke risk in atrial fibrillation and in people with mechanical heart valves.
INR and your yellow book
Warfarin dosing is guided by the INR blood test, which measures how long your blood takes to clot. Your target range is set for your condition, commonly around 2 to 3.
Take the dose your anticoagulant clinic tells you, at the same time each day, usually early evening. The dose can change between tests — always follow the most recent instruction rather than a habit.
Keep your yellow anticoagulant book with you and take it to every appointment, including dentists, opticians and hospital visits. Tell any healthcare professional you take warfarin before any procedure.
If you miss a dose, take it the same day if you remember. If you only remember the next day, skip it — never double up — and note it for the clinic.
What changes your INR
Vitamin K in food — green leafy vegetables, broccoli, sprouts. You do not need to avoid them; you need to keep intake consistent rather than swinging between none and lots.
Alcohol — keep it steady and moderate; binge drinking destabilises the INR considerably.
Cranberry juice and large quantities of grapefruit.
Illness, particularly vomiting, diarrhoea or fever.
Many medicines, including most antibiotics, anti-inflammatories, aspirin, some antidepressants, and herbal remedies such as St John’s wort.
Always check with us before taking any new medicine, including anything bought over the counter or herbal. Ibuprofen and aspirin in particular add bleeding risk on top of warfarin.
Bleeding — what to watch for
Contact your doctor or anticoagulant clinic promptly for:
Bleeding that will not stop, or unusually heavy periods
Blood in urine, or black tarry stools
Coughing or vomiting blood
Unexplained widespread bruising
Severe headache, or any head injury — even a minor bump warrants advice
Call 999 for severe bleeding, a serious head injury, or sudden severe headache with confusion or weakness.
Warfarin must not be taken in pregnancy. Speak to your GP promptly if you are pregnant or planning a pregnancy.
Always read the label. This page is general information, not advice for your situation. Doses, warnings and interactions differ between products and between people. Check the leaflet inside your medicine, and ask our pharmacist if anything is unclear — that conversation is free and takes two minutes.
Questions about Warfarin?
1 Lyng Lane, West Bromwich B70 7RW · Mon–Fri 8:30am–6pm, Sat 9am–1pm · 0121 500 5756
No — you need to keep your intake consistent rather than avoid them. Big swings in vitamin K are what destabilise the INR.
Can I take ibuprofen with warfarin?
Check with us first. Anti-inflammatories and aspirin add substantially to bleeding risk on top of warfarin, and paracetamol is usually the safer painkiller.
What should I do if I miss a warfarin dose?
Take it the same day if you remember. If you only remember the next day, skip it, never double up, and tell your clinic.
When is bleeding on warfarin an emergency?
Severe bleeding that will not stop, coughing or vomiting blood, black stools, a head injury, or sudden severe headache with confusion — call 999.
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