Warfarin
Sold as Coumadin, Jantoven
Warfarin is an oral anticoagulant ("blood thinner") that prevents dangerous blood clots in conditions like atrial fibrillation, DVT/PE, and mechanical heart valves. It requires regular blood testing (INR) to keep the dose safe and effective.
Boxed warning
BOXED WARNING: Warfarin can cause major or fatal bleeding, including gastrointestinal and intracranial (brain) hemorrhage. Bleeding risk is highest when starting therapy and when the INR is above the target range. Regular INR monitoring is required for the life of the prescription — never skip lab draws. Seek emergency care for severe headache, black/bloody stool, coughing/vomiting blood, or unusual bruising/bleeding.
What it treats
Prevention and treatment of blood clots (deep vein thrombosis, pulmonary embolism), stroke prevention in atrial fibrillation, and clot prevention in patients with mechanical heart valves.
Typical dosing
Mechanism: inhibits vitamin K epoxide reductase, which blocks activation of clotting factors II, VII, IX, and X, and proteins C and S. Dosing is NOT one-size-fits-all — it is individualized entirely by INR response: • Typical starting dose: 2–5 mg once daily (many patients end up on 2–10 mg/day). • Elderly, malnourished, or liver-impaired patients: start LOWER (often 2.5 mg or less) — they're more sensitive to warfarin's effect. • Target INR: 2.0–3.0 for most indications (afib, DVT/PE); 2.5–3.5 for a mechanical mitral valve or certain high-risk mechanical valves — your prescriber sets your specific target. • Dose is adjusted in small increments (5–20%) based on INR trend, never by guesswork.
Monitoring
INR (International Normalized Ratio) blood test: checked every 2–3 days when starting or adjusting dose, then weekly, then extended to every 4 weeks (every 4–6 weeks once stable). More frequent checks are needed after any change in diet, illness, new medication, or missed doses.
Side effects
Bleeding is the primary risk (see boxed warning) — from minor bruising and gum bleeding to serious GI or intracranial hemorrhage. Rare: skin necrosis (usually in the first days of therapy, more common with protein C/S deficiency), "purple toe syndrome."
Interactions to know
Warfarin has dozens of clinically significant interactions — always use a drug-interaction checker or ask your pharmacist before starting or stopping ANY medication, including OTC and herbal products. Notable examples that increase bleeding risk or INR: amiodarone, trimethoprim-sulfamethoxazole (TMP-SMX/Bactrim), fluconazole and other azole antifungals, macrolide antibiotics (azithromycin, clarithromycin, erythromycin), NSAIDs (ibuprofen, naproxen), aspirin, and many others. St. John's wort and some other supplements can lower warfarin's effect and increase clotting risk.
Pregnancy & breastfeeding
Warfarin is teratogenic and can cause fetal warfarin syndrome (nasal hypoplasia, skeletal and CNS abnormalities) with exposure in the first trimester, and fetal bleeding risk later in pregnancy. It is contraindicated in pregnancy EXCEPT in specific situations your cardiologist/OB may decide together (e.g. certain mechanical heart valves where alternatives carry unacceptable clotting risk). Anyone who could become pregnant should discuss contraception and alternative anticoagulants with their prescriber.
Additional notes
Reversal (only under medical supervision): vitamin K (phytonadione) for elevated INR or bleeding; 4-factor prothrombin complex concentrate (PCC) is the preferred rapid-reversal agent for serious bleeding or urgent surgery (faster and more complete than plasma); fresh frozen plasma (FFP) is an alternative when PCC isn't available. Dietary: keep vitamin-K intake (leafy greens, etc.) CONSISTENT week to week rather than avoiding it — sudden increases or decreases both throw off INR control. Special populations: elderly patients need lower starting doses and closer monitoring due to increased sensitivity and interacting medications.
Clinical content reviewed by the BetterBuyRx clinical team, on 2026-07-07.
Educational only. Not medical advice. Always confirm with your prescriber or pharmacist.