Warfarin
- Class: Vitamin K antagonist
- Used for: blood clots
- Say it: WOR-fuh-rin
- Forms: tablet
WARFARIN wages WAR on the clotting factors — slowly, one supply line at a time.
It blocks vitamin K recycling, so the liver cannot build factors II, VII, IX and X. Cutting supply lines takes days, which is why warfarin is slow to start and slow to stop.
It blocks vitamin K recycling, so factors II, VII, IX and X get made but do not work. Eating a steady amount of green vegetables matters far more than avoiding them.
Side effects
Common: bruising, nosebleeds, bleeding gums
Serious: major bleeding, skin necrosis (rare)
Typical dose
2–10 mg once daily, set by the INR
There is no standard dose — the INR decides it. Target 2–3 for most indications, 2.5–3.5 for mechanical valves.
Check yourself
Why does Warfarin take two to three days to work?
- The clotting factors already circulating have to be used up first
- It is absorbed slowly from the gut
- The kidney has to convert it into its active form
- Vitamin K stores have to be eaten through first
Show answer
A. The clotting factors already circulating have to be used up first
Warfarin stops new working factors being made; it does nothing to the ones already in the blood. The delay is simply how long those take to be used up — which is why heparin covers the gap at the start.
See it, don't memorize it
The app has the mechanism comic and a patient case for Warfarin – plus the name link, the class mnemonic, pronunciation you can hear, and a self-check, for the top 100 drugs. One purchase, no subscription, no account.