Warfarin — NZ Medication Guide | KiwiMeds
✅ Reviewed by a Registered Pharmacist NZ | Last updated: May 2026 | This information is for educational purposes only and does not replace advice from your doctor or pharmacist.
What is Warfarin?
Warfarin is an anticoagulant medicine — commonly called a “blood thinner.” It reduces the ability of blood to clot, helping to prevent and treat blood clots. Warfarin has been used for decades and is very effective when managed properly, though it requires regular blood test monitoring.
What is it used for?
Warfarin is used to prevent and treat blood clots in conditions including deep vein thrombosis (DVT), pulmonary embolism (PE), atrial fibrillation (to prevent stroke), and mechanical heart valves. It is fully funded by Pharmac. Note: For atrial fibrillation and VTE, the newer direct oral anticoagulants (DOACs — apixaban, rivaroxaban) are often preferred over warfarin as they do not require monitoring and have fewer interactions.
How does it work?
Warfarin blocks the action of vitamin K in producing clotting factors. This means it slows the formation of blood clots. Its effect is measured by the INR (International Normalised Ratio) — a blood test. The target INR depends on the condition being treated, most commonly 2.0–3.0.
How to take it
Warfarin is taken once daily at the same time each day. The dose is individualised based on INR results and is adjusted regularly. Consistency in diet (especially vitamin K intake) is important. Take exactly as prescribed — even small dose changes affect INR significantly.
Common side effects
Warfarin itself has few direct side effects at therapeutic doses, but the main risk is bleeding. Signs of minor bleeding include easy bruising, nosebleeds, bleeding gums, heavier periods, and prolonged bleeding from cuts.
Serious side effects to watch for
Seek immediate medical care for signs of major bleeding: blood in urine (pink/red), blood in stools (black or tarry), vomiting blood, coughing up blood, unusual or severe headache, severe abdominal pain, or any injury to the head.
Important drug interactions
Warfarin has more drug interactions than almost any other medicine. Key interactions: antibiotics (many affect INR — always tell prescribers you take warfarin), NSAIDs (aspirin, ibuprofen — increase bleeding risk), amiodarone (greatly increases INR), statins, and herbal remedies including St John’s Wort. Many foods affect INR — particularly vitamin K-rich foods like leafy greens. ALWAYS tell every doctor, dentist, and pharmacist that you take warfarin.
Things to avoid
Avoid NSAIDs — use paracetamol for pain instead. Avoid vitamin K supplements. Maintain a consistent vitamin K intake (do not suddenly increase or decrease leafy greens). Avoid excessive alcohol. Avoid contact sports or activities with high fall/injury risk.
NZ-specific information
Warfarin is funded by Pharmac. INR monitoring is available through community labs — funded by the health system. Many GP practices and pharmacies offer warfarin management services. Some areas have community anticoagulation clinics. Carry a warfarin card and wear a medical ID bracelet.
Frequently asked questions
How often do I need INR tests? When starting or adjusting, tests may be weekly. When stable, most people test every 4–8 weeks. What foods affect warfarin? Vitamin K-rich foods (kale, spinach, broccoli) can lower INR — consistency is the key, not avoidance. Can I switch from warfarin to apixaban? Discuss with your doctor — for AF, most people can switch to a DOAC with guidance.
💬 Always talk to your pharmacist or doctor for advice specific to you. This guide is for general information only and does not replace a professional consultation.
Related medications
Related: Apixaban, Clopidogrel. Condition: Atrial Fibrillation.
References & Further Information
The following New Zealand and international resources were used to inform this page: