What it is used for
- Preventing and treating blood clots in the legs or lungs
- Reducing stroke risk in atrial fibrillation
- Preventing clots in people with mechanical heart valves
- Preventing clots after certain surgeries
How it works
Warfarin reduces the blood's ability to form clots by interfering with vitamin K, which the body needs to make several clotting factors. This helps prevent dangerous clots, but it requires careful monitoring to keep the effect in a safe range.
How it is taken
Taken by mouth as a tablet, usually once a day at the same time, often in the evening. The amount is adjusted based on a regular blood test (called the INR). Take it exactly as your doctor or pharmacist directs and follow the label; keep a consistent diet and attend all monitoring appointments.
No doses here on purpose. The right strength and schedule depend on you — only a doctor or pharmacist should set them. Always follow your prescription and the label.
Common side effects
- Easier bruising
- Bleeding gums or nosebleeds
- Longer bleeding from minor cuts
- Heavier menstrual bleeding
Serious effects — get help
- Heavy or uncontrolled bleeding
- Black or bloody stools, or vomiting blood or material that looks like coffee grounds
- Coughing up blood, blood in the urine, or sudden severe headache
- Unusual or severe bruising, or painful skin changes
- Signs of a clot despite treatment, such as leg swelling or sudden breathlessness
If a reaction is severe — for example trouble breathing, swelling of the face or throat, or collapse — go to the nearest emergency room now or call your local emergency number. Do not wait for an online consultation.
Interactions to watch
- Many other medicines including antibiotics, antifungals, and some painkillers
- Other blood thinners and antiplatelets such as aspirin and clopidogrel
- Non-steroidal anti-inflammatory drugs such as ibuprofen
- Some herbal products and supplements (e.g. St John's wort, high-dose fish oil)
- Foods high in vitamin K such as large amounts of leafy greens
- Alcohol, especially binge drinking
Who should take care
- Do NOT use in pregnancy unless specifically directed — it can harm the unborn baby; discuss with your doctor
- Keep your vitamin K intake (leafy greens) steady rather than changing it suddenly
- Tell every clinician, dentist, and pharmacist that you take warfarin
- Attend all INR blood-test appointments
- Check with a pharmacist before starting any new medicine or supplement