The long-established oral anticoagulant that blocks vitamin K-dependent clotting factors; effective for mechanical heart valves and clots but needs regular INR blood tests and careful dosing.
Legal status differs between countries and can change; each row cites the document it was taken from and applies only to the country named.
What it is used for
Mechanical heart valves (the only oral anticoagulant licensed)
Atrial fibrillation with moderate or severe mitral stenosis, and antiphospholipid syndrome
Venous thromboembolism where DOACs are unsuitable (severe kidney failure, cost)
Historically the standard anticoagulant, now largely replaced by DOACs for other indications
How it works in the body
Blocks vitamin K epoxide reductase in the liver, so vitamin K cannot be recycled to activate clotting factors II, VII, IX and X. Existing factors take days to be cleared, so the effect starts after 2-3 days and heparin cover is needed for immediate protection. The dose needed varies enormously between people and with diet, illness and other drugs, so the INR blood test tunes it.
Common side effects
Bruising, bleeding gums, nosebleeds
Hair thinning, rash
Serious: seek help
Major bleeding, including intracranial: risk rises steeply when the INR is above 4
Skin necrosis in the first days (protein C deficiency), purple toe syndrome
Fetal harm: teratogenic in the first trimester and bleeding later
Cautions and interactions
Hundreds of interactions: antibiotics, antifungals, amiodarone, NSAIDs, alcohol binges raise the INR; rifampicin, carbamazepine, large amounts of green vegetables lower it
Keep vitamin K intake steady rather than avoiding it
Carry an anticoagulant alert card; tell dentists and pharmacists
Target INR 2-3 for most indications, 2.5-3.5 for some mechanical valves
Interactions
Check interactions
Taking Warfarin with other medicines? Add them to the checker to see what the official sources say about each pair: the mechanism, general management and monitoring, never a bare "safe".
Interactions documented in the official sources cited on each card. The list covers the medicines represented on this site and is not exhaustive; how the interaction data is compiled.
Aspirin with warfarin substantially increases bleeding, including gastrointestinal and intracranial bleeding.
Why it can occur
additive bleeding risk
What official information says
UK information lists blood thinners such as warfarin and apixaban among medicines to tell a doctor about; the BNF advises avoiding the combination unless specifically indicated.
What to discuss with a clinician
Official guidance: combined only for specific indications (for example mechanical valves) under specialist direction, usually with gastroprotection.
NHS: Amoxicillin(Taking it with other medicines and herbal supplements · Medicines that interact with amoxicillin)UK BNF: Amoxicillin interactions(Interactions)UK NHS: Warfarin(Taking it with other medicines and herbal supplements · Medicines that interact with warfarin)UK
Review
Facts checked against the cited sources; not yet clinically reviewed · last reviewed 12 September 2026
Therapeutic duplication. Two anticoagulants together (for example warfarin with a DOAC outside a supervised switch) multiply bleeding risk and are avoided. UK
Apixaban and DOACsapixabandabigatrandalteparinedoxabanenoxaparinrivaroxaban
Medicines that share the class of Warfarin. They are separate medicines with their own licences, doses and interactions, not alternative names for Warfarin.
Medicines often discussed alongside Warfarin: used together, compared with it, or acting on the same problem by a different route.
Educational content. Anatomy Nexus provides medical education and does not replace professional medical advice, diagnosis, treatment, prescribing or pharmacist review. Doses, choices and monitoring are decided by a prescriber for an individual; never start, stop or change a medicine on the basis of this page.
