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AnticoagulantBlood thinner · VKA

Warfarin (Coumadin, Jantoven)

A highly effective blood thinner that has been in use since the 1950s. Requires regular INR monitoring and interacts with many foods, drugs, and supplements — but remains a preferred option for several specific conditions.

What it's used for

Preventing and treating blood clots: atrial fibrillation, deep vein thrombosis, pulmonary embolism, mechanical heart valves, certain inherited clotting disorders.

Typical dosing & monitoring

  • Dose: individualized; typical range 2-10mg once daily.
  • INR target: usually 2.0-3.0 (2.5-3.5 for mechanical valves).
  • Monitoring: INR checks weekly at start, then every 4-12 weeks once stable.
  • Timing: same time each evening is conventional.

Key interactions (short list — it's extensive)

  • NSAIDs (ibuprofen, naproxen): major — bleeding risk.
  • Aspirin: major — bleeding risk; sometimes co-prescribed intentionally with careful monitoring.
  • Antibiotics (especially metronidazole, TMP-SMX, ciprofloxacin): major — raise INR significantly.
  • Amiodarone: major — large INR rise; warfarin dose typically cut.
  • SSRIs: moderate — bleeding risk.
  • Acetaminophen (≥2g/day regularly): moderate — can raise INR.
  • Alcohol (acute binge): raises INR; chronic heavy use lowers it.
  • Supplements: vitamin K (lowers INR), St. John's Wort (lowers), ginkgo, garlic, fish oil (raise bleeding risk).

Diet & vitamin K

Vitamin K counteracts warfarin. You don't need to avoid leafy greens — you need to keep your intake consistent. Sudden large changes (new kale habit, or stopping salads) can swing your INR.

Common side effects

Easy bruising, prolonged bleeding from small cuts, occasional bleeding gums. Serious: GI bleeding, urinary bleeding, major bleeding (especially intracranial) — seek emergency care for severe headache, black stools, or heavy bleeding that won't stop.

Who should be cautious

  • Pregnancy (contraindicated, especially first trimester).
  • High fall risk, unstable housing, or inability to attend INR monitoring.
  • Heavy alcohol use.

What to ask your pharmacist

  • Would a DOAC (apixaban, rivaroxaban) be appropriate for me instead?
  • Should I carry a wallet card identifying me as on warfarin?
  • What should I do before dental work or surgery?
  • Any new medication or supplement — does it interact?