What is the normal dose for warfarin (International Normalized Ratio - INR)?

Warfarin Dosing Guidelines

For most patients, warfarin should be initiated at 5 mg daily, which produces therapeutic anticoagulation within 4-6 days, with the target INR maintained at 2.0-3.0 for most indications. 1, 2

Initial Dosing Strategy

Standard starting dose:

• Start with 5 mg daily for most outpatients, which achieves satisfactory anticoagulation in 4-6 days 1, 2
• The FDA label supports individualized dosing but emphasizes that most patients respond well to this standard approach 2

Reduced starting doses (2-4 mg daily) are indicated for: 1, 3

• Elderly patients (≥65 years)
• Debilitated patients
• Patients at increased bleeding risk
• Those expected to have heightened warfarin sensitivity

Alternative loading approach:

• A 10 mg daily dose for 2 days achieves therapeutic INR faster (4.2 vs 5.6 days) but shows no difference in clinical outcomes (bleeding, thromboembolism, mortality) 3
• The American College of Chest Physicians notes that faster INR achievement does not translate to improved morbidity or mortality 3

Target INR Ranges by Indication

Most common indications (INR 2.0-3.0): 2, 4

• Venous thromboembolism (DVT/PE)
• Atrial fibrillation (non-valvular)
• Post-myocardial infarction (moderate-intensity regimen)

Higher intensity (INR 2.5-3.5): 2, 4

• Mechanical prosthetic heart valves (bileaflet or tilting disc)
• Caged ball or caged disc valves may require even higher targets

Highest intensity (INR 3.0-4.0): 2

• High-risk post-MI patients in settings with meticulous INR monitoring

Maintenance Dosing

Typical maintenance dose range:

• Most patients require 2-5 mg daily (approximately 80% of patients achieve stable anticoagulation in this range) 5
• Enormous interpatient variability exists: some patients need as little as 0.5-2.0 mg/day while others require >80 mg/week 1
• Elderly patients typically require approximately 1 mg/day less than younger patients 1

The target is INR, not dose - continue adjusting until therapeutic INR is achieved and maintained, regardless of final dose required 1

INR Monitoring Schedule

Initial phase: 1, 3

• Check INR daily until therapeutic range is reached and sustained for 2 consecutive days
• When rapid anticoagulation is needed, give heparin concurrently for at least 4 days until INR is therapeutic for 2 consecutive days

Stabilization phase: 6, 1

• Monitor 2-3 times weekly for 1-2 weeks
• Then weekly for 1 month

Maintenance phase: 6, 1

• Once stable, testing can be reduced to intervals as long as 4 weeks (some guidelines suggest up to 4-6 weeks maximum)
• More frequent monitoring required during medication changes, illness, or dietary fluctuations

Management of Elevated INR

INR <5.0 without bleeding: 3

• Reduce or omit next dose, resume at lower dose when INR approaches therapeutic range

INR 5.0-9.0 without bleeding: 6, 3

• Omit 1-2 doses and reinstate at lower dose
• For patients at increased bleeding risk: give 1.0-2.5 mg oral vitamin K

INR >9.0 without bleeding: 6, 3

• Give 3-5 mg oral vitamin K
• Oral vitamin K (1-2.5 mg) effectively reduces INR within 24 hours and is preferred over parenteral routes due to safety 6

Serious bleeding at any INR: 3

• Give 5-10 mg IV vitamin K
• Administer fresh-frozen plasma or prothrombin concentrate for immediate reversal

Special Population: Pregnancy with Mechanical Valves

Low-dose warfarin (≤5 mg/day): 6, 1

• Continuation throughout all 3 trimesters is reasonable after informed consent
• Risk of warfarin embryopathy remains low (<3%) at this dose

High-dose warfarin (>5 mg/day): 6, 1

• Switch to dose-adjusted LMWH (target anti-Xa 0.8-1.2 U/mL at 4-6 hours post-dose, twice daily) during first trimester
• Resume warfarin for second and third trimesters
• Risk of fetal loss or embryopathy exceeds 30% if continued during first trimester

Peripartum management: 6

• Switch from warfarin to LMWH or UFH at least 1 week before planned delivery
• If labor begins while therapeutically anticoagulated, perform cesarean section after reversal

Critical Pitfalls to Avoid

Early INR elevation does not equal full anticoagulation: 3

• Initial INR rise reflects depletion of factor VII and protein C, not complete anticoagulant effect
• Full anticoagulation requires factor II depletion, which takes 4-5 days regardless of initial dosing
• Do not discontinue heparin prematurely based on early INR elevation alone

Bleeding risk increases exponentially: 4

• Risk becomes clinically unacceptable when INR exceeds 5.0
• Elderly patients have higher bleeding risk at any given INR level 6

Avoid assuming a "maximum safe dose" exists: 1

• Some patients legitimately require doses >10 mg/day due to genetic polymorphisms (CYP2C9, VKORC1)
• The appropriate dose is determined by INR response, not an arbitrary ceiling

High-dose vitamin K causes warfarin resistance: 3

• Doses of 10 mg may cause resistance for up to a week
• Use lower doses (1-2.5 mg oral) for non-emergent INR correction 6