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Warfarin

Brand names: Jantoven

Warfarin is a blood thinner that prevents blood clots. It needs regular blood tests (INR) to keep the dose right.

Brand namesJantoven
Type of medicineBlood thinner (anticoagulant)
Prescription?Prescription
ImportantThis page is general information, not a prescription. Don't start, stop, or change a medicine without talking to your provider or pharmacist.

Overview

It blocks vitamin K, which the body needs to make clotting factors.

The right dose differs a lot from person to person and can change with diet, illness, and other medicines.

What it's used for

  • Lowering stroke risk in atrial fibrillation
  • Treating and preventing blood clots in the legs and lungs
  • Preventing clots with mechanical heart valves

How to take it

  • Take it once a day at the same time.
  • Keep all your INR blood test appointments.
  • Keep the amount of vitamin K foods (leafy greens) steady from week to week rather than avoiding them.
  • Tell every provider and dentist you take it.

Your dose depends on your health and other medicines. Always follow your prescription label, and ask your pharmacist if anything is unclear.

Side effects

Common side effects

  • Bruising easily
  • Minor bleeding
Call 911Vomiting or coughing up blood, or bleeding that won't stop
Call 911A severe headache, weakness, confusion, or trouble speaking
Call 911A bad fall or hit to the head
Call your provider todayRed or brown urine; black or bloody stools
Call your provider todayPainful purple toes or skin turning dark

Warnings

  • Many foods, drinks, and medicines change how warfarin works. Check before starting anything new, including supplements.
  • Avoid binge drinking.

Other medicines, foods, and alcohol

  • Many antibiotics, antifungals, and heart medicines (like amiodarone) raise its effect.
  • Aspirin, NSAIDs, and SSRIs raise bleeding risk.
  • Supplements like fish oil, ginkgo, garlic, and St. John's wort can change its effect.
  • Large changes in leafy green vegetables or cranberry juice can change your INR.

Tell your provider and pharmacist about every medicine, vitamin, and supplement you take.

Pregnancy and breastfeeding

Warfarin can cause birth defects. Don't take it in pregnancy unless you have a mechanical heart valve and your provider directs it.

Missed doses, overdose, and storage

If you miss a dose

Take it as soon as you remember, unless it's almost time for your next dose. Then skip the missed dose. Don't take two doses at once.

If someone takes too much

Call Poison Control at 1-800-222-1222 or get help online at poison.org. Call 911 if the person collapses, has a seizure, has trouble breathing, or can't be woken.

Storing it

Store at room temperature, away from heat and moisture (not in the bathroom), and out of reach of children and pets.

Conditions it's used for

Sources

Last updated 2026-11-13.