Warfarin
(Redirected from Warfarin (Coumadin))
General
- Type: anticoagulant
- Dosage Forms: oral
- Common Trade Names: Coumadin
Adult Dosing
- Day 1: 5 - 7.5mg oral at bedtime (to ensure absorption on empty stomach)
- Day 2 and beyond: 2.5 - 7.5mg daily (usually 5mg)
- INR increase of >0.3 - 0.4 per day requires dose reduction
- Response also influenced by congestive heart failure, liver disease, Vitamin K deficiency, many drugs
Target Range of INR
- 2.5 - 3.5: Mechanical prosthetic valves or recurrent thromboembolism
- 2.0 - 3.0: All other indications
Pediatric Dosing
Safety and efficacy not well-established, though it is sometimes used in pediatrics for VTE treatment
Special Populations
- Pregnancy Rating: Contraindicated (Category D)
- Lactation risk: infant risk minimal
- Renal Dosing: caution advised
- Hepatic Dosing: consider decreased dose
Contraindications
- Allergy to class/drug
- Active bleeding or tendency to bleed
Adverse Reactions
- Bleeding
- Risk increased when INR >3
- Exponential increase when INR >5
- Avoid giving NSAIDs, sulfas, macrolidies (azithromycin ok), fluoroquinolones
- Skin necrosis
- Usually in patient with protein C deficiency
- Occurs 3 - 8 days after starting treatment
- Treatment: see Warfarin (Coumadin) Reversal
Pharmacology
- Half-life: 20-60h (anticoagulant effect)
- Of note, half-life highly variable based on rate of clotting factor catabolism
- Metabolism: Liver
- Excretion: Urine 92%, bile
- Mechanism of Action:
- Blocks activation of vitamin K dependent prothrombotic factors II, VII, IX, X
- Blocks synthesis of vitamin K dependent antithrombotic proteins C and S
- Albumin bound
Comments
- Because of differing factor half-lives, can cause transient (24-36 hour) prothrombosis at start of therapy
- Bridge with heparin for 1-2 days until INR is in desired range