Warfarin

COUMADIN

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Pronunciation: WAR-far-in (KOO-ma-din)

Warfarin ballicule: receptor binding, kinetics and half-life diagram — Mechanism of action: Warfarin ballicule

Warfarin COUMADIN mascot mnemonic cartoon — Visual mnemonic: Warfarin mascot
**“Warfare Comin’ down”**
Vitamin K is warfarin mechanism/antidote

What it isVitamin K antagonist blocking factors II, VII, IX and X; mechanical valves, atrial fibrillation and VTE.

Why this oneIt remains the oral anticoagulant for mechanical prosthetic valves and is readily reversed with vitamin K and factor concentrate.

What limits itNarrow therapeutic index with INR monitoring, CYP2C9 and VKORC1 variation, and interactions with diet, antibiotics and amiodarone.

Warfarin molecular structure (skeletal formula) — Molecular structure: Warfarin

FDA-approved for

thromboembolism prophylaxis/treatment (1954)

FDA label

Read the full label on DailyMed ↗

BOXED WARNING Can cause major or fatal bleeding; monitor INR regularly

FDA dosing	Population	Start	Target	Max
Thromboembolism prophylaxis/treatment (VTE/DVT/PE, atrial fibrillation, post-MI, valve replacement)	adult	2-5 mg QD (no genotype)	2-10 mg QD, adjust to INR (target INR 2.5 [2.0-3.0]; 3.0 [2.5-3.5] for mechanical mitral/caged valves)	INR-guided (INR >4.0 no added benefit)

Renal No adjustment needed; monitor INR more frequently

Hepatic Response may be potentiated; monitor more frequently for bleeding

Routine loading doses not recommended

Instructions

Individualize dose to INR; obtain daily INR until stable, then every 1-4 weeks
Missed dose: take same day; do not double the next day
When converting from heparin, overlap 4-5 days until INR therapeutic
Interrupt/adjust for surgery per INR; consider lower doses in elderly/debilitated/Asian patients

Available strengths

tab 1, 2, 2.5, 3, 4, 5, 6, 7.5, 10 mg

Contraindications

Pregnancy (except women with mechanical heart valves at high thromboembolism risk)
Hemorrhagic tendencies or blood dyscrasias
Recent or contemplated CNS or eye surgery, or traumatic surgery with large open surfaces
Active bleeding (GI/GU/respiratory tract, CNS hemorrhage, cerebral aneurysm, dissecting aorta, pericarditis/effusion, bacterial endocarditis)
Threatened abortion, eclampsia, preeclampsia
Unsupervised patients with high potential for non-compliance
Spinal puncture / procedures with potential for uncontrollable bleeding
Major regional or lumbar block anesthesia
Malignant hypertension

Cautions

Tissue necrosis/gangrene
Calciphylaxis
Acute kidney injury during excessive anticoagulation/hematuria
Systemic atheroemboli and cholesterol microemboli (purple toes syndrome)
Do not use as initial therapy in HIT/HITTS
Protein C/S deficiency
Fetal harm in pregnancy

Adverse reactions

Fatal and nonfatal hemorrhage from any tissue/organ
Tissue necrosis
Hypersensitivity/allergic reactions
Hepatitis/elevated liver enzymes

Pregnancy Contraindicated except in women with mechanical heart valves; causes warfarin embryopathy, fetal hemorrhage, fetal death

Lactation Not detected in human milk; monitor breastfed infants for bruising or bleeding

Cost

Warfarin (COUMADIN)

What pharmacies pay · NADAC 9/30/26

Strength		#/mo	generic $/mo	CostPlus
1 mg	tab	30 qd	$2.96	offered
2 mg	tab	30 qd	$2.79	offered
2.5 mg	tab	30 qd	$2.88	offered
3 mg	tab	30 qd	$2.96	offered
4 mg	tab	30 qd	$2.87	offered
5 mg	tab	30 qd	$2.66	offered
6 mg	tab	30 qd	$2.94	offered
7.5 mg	tab	30 qd	$2.67	offered
10 mg	tab	30 qd	$3.19	offered

Generic entry1982

⚠ BOXED WARNING

Bleeding risk

Classes and tags

Clinical profile

Therapeutic safety burden3 / 4

Overdose danger4 / 4

Weight-gain liability0 / 4

Sedation liability0 / 4

QT / Torsades0 / 4

Direct muscarinic antagonism0 / 4

Embryofetal risk4 / 4

Legacy pregnancy categoryX

Defining liabilityMajor bleeding with extensive food, disease and drug interactions; INR monitoring is required.

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Memory aids: interaction avatars

CYP2C19 substrate

CYP2C9 substrate

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