Warfarin Administration
Warfarin is administered orally as a tablet, typically once daily, with an initial dose of 4-5 mg for most patients (or 2-4 mg in elderly patients), followed by dose adjustments based on INR monitoring. 1, 2
Route of Administration
The standard and FDA-approved route for warfarin is oral administration. The drug is essentially completely absorbed after oral administration, with peak concentrations reached within 4 hours 2. Warfarin tablets are taken by mouth, typically at the same time each day to maintain consistent anticoagulation.
Alternative Routes (Non-Standard)
While oral administration is standard, alternative routes have been reported in specific clinical scenarios:
- Intravenous warfarin: Available for patients who cannot take oral medications (e.g., severe malabsorption from Crohn's disease), though this is not commonly used 3
- Sublingual administration: Case reports describe successful sublingual dosing in patients with gastrointestinal dysmotility, achieving therapeutic INR in approximately 6 days 4
These alternative routes should only be considered when oral administration is truly impossible, as they lack extensive safety and efficacy data.
Dosing Strategy
Initial Dosing
Start with 5 mg daily for most patients 1. This approach is superior to loading doses:
- Avoid loading doses >5 mg, which cause excessive early INR elevation without improving time to therapeutic anticoagulation 1, 5, 6
- Use 2-4 mg daily in elderly patients (>75 years), who require approximately 1 mg/day less than younger patients 1
- Use lower initial doses (4-5 mg) in patients at increased bleeding risk 1
The rationale: A 5-mg loading dose produces less excess anticoagulation than 10-mg doses and avoids precipitous drops in protein C that can create a transient hypercoagulable state in the first 36 hours 6.
Monitoring Schedule
- Days 1-2: Check INR daily until therapeutic range achieved for 2 consecutive days 1
- Weeks 1-2: Check INR 2-3 times weekly 1
- Stable patients: Gradually extend intervals up to every 4 weeks maximum 1
- After dose adjustments: Resume frequent monitoring 1
Dose Adjustments
When adjusting warfarin doses at steady state:
- Do not adjust for a single slightly out-of-range INR 5
- Modify total weekly dose by 5-20% for most adjustments 5
- Target INR range is typically 2.0-3.0 for most indications 1
Important Clinical Considerations
Concurrent heparin bridging: When rapid anticoagulation is needed, administer heparin concurrently with warfarin for 4 days. Heparin can be discontinued once INR has been therapeutic for 2 consecutive days 1.
Pharmacogenetic factors: Patients with CYP2C92 or CYP2C93 variant alleles have decreased warfarin clearance and require lower doses (17-37% less than wild-type patients) 2. Consider pharmacogenetic-guided dosing when available, particularly for loading doses 7.
Onset of effect: Anticoagulation begins within 24 hours, but peak effect is delayed 72-96 hours due to the varying half-lives of vitamin K-dependent clotting factors 2.
Common Pitfalls
- Avoid 10-mg loading doses: These cause excessive INR elevation (44% of patients >2.0 at 36 hours vs. 8% with 5-mg doses) and increase risk of over-anticoagulation 6
- Don't start outpatient therapy without a plan: For non-urgent anticoagulation (e.g., chronic atrial fibrillation), warfarin can be initiated as an outpatient with 4-5 mg daily, achieving therapeutic effect within 6 days 1
- Remember protein binding: Warfarin is 99% protein-bound, making drug interactions particularly important 2