Warfarin

Key facts

• Warfarin is an anticoagulant (blood thinner). It slows the liver's production of clotting proteins so blood clots form less easily.
• You must have regular INR blood tests, often weekly at first and monthly once your dose is stable. The test tells your prescriber whether your dose is too low, too high, or right.
• Warfarin has a narrow margin between too little effect (clots) and too much (bleeding). Keep vitamin K intake (kale, spinach, leafy greens) consistent, and tell every prescriber and pharmacist you take it, since many common drugs interact with it.
• Seek urgent care for black or bloody stools, blood in urine or vomit, a sudden severe headache, or a fall or head injury.

What warfarin treats

Warfarin prevents and treats blood clots. It is used for atrial fibrillation to prevent stroke, for deep vein thrombosis and pulmonary embolism, and for people with mechanical heart valves, where it remains the preferred anticoagulant. It does not dissolve an existing clot; it stops new clot material from forming while the body clears the old one on its own.

How warfarin works

Your liver uses vitamin K to build several of the proteins that make blood clot. Warfarin blocks the enzyme that recycles vitamin K, so those clotting proteins are made in a far less active form. The effect takes several days to build up and several days to wear off after you stop, because existing clotting proteins already in your blood have to clear first.

Before you take it

• Do not start warfarin if you are pregnant; it can cause birth defects and bleeding in the fetus. Tell your prescriber if you could become pregnant.
• Tell your prescriber about liver or kidney disease, any bleeding disorder, recent surgery or fall risk, and every other medicine or supplement you take, including antibiotics, anti-inflammatory painkillers, and herbal products like St John's wort.
• Keep your diet's vitamin K content steady from week to week rather than avoiding it; a sudden change, not the vegetables themselves, is what throws your INR off.
• Alcohol changes how warfarin is broken down; keep intake consistent and tell your prescriber how much you drink.

Side effects

Common effects include easy bruising, minor nosebleeds or bleeding gums, and small pinprick spots of bleeding under the skin.

Stop and seek urgent medical care for any of these:

• Bleeding that will not stop, black or tarry stools, or blood in urine or vomit.
• A sudden, severe headache, confusion, or weakness on one side of the body.
• A significant fall or head injury, even without obvious bleeding.
• Unusual, large or spreading bruises.

Safety essentials

• Attend every scheduled INR blood test. This is not optional monitoring: it is how your prescriber keeps you inside the narrow safe range between clotting and bleeding.
• Keep vitamin K intake steady and disclose every new medicine, supplement or antibiotic course before you start it, since many interact with warfarin and shift your INR.
• Carry an anticoagulant alert card and tell any doctor, dentist or emergency team that you take warfarin before any procedure or new prescription.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.