Warfarin adult dose is 5mg, oral, daily adjusted to maintain INR range between 2 and 3.
Warfarin Interactions
A large number of medicines interact with warfarin leading to under- or over-anticoagulation, and careful evaluation of all new medicines, herbal and over-the counter products is critical. This includes (but is not an exhaustive list):
- Medicines altering platelet function e.g. NSAIDs, aspirin, clopidogrel, etc.
- Foods or medicines altering vitamin K synthesis e.g. antibiotics, rifampicin, macrolides, simvastatin, phenytoin, carbamazepine, etc.
- Grapefruit juice
Unless INR Is markedly out of range the modest adjustments recorded below should be followed.
Warfarin Initiation
Warfarin initiation dosing protocol (week 1) with INR target: 2-3 is as follows.
| Day Therapy | INR value | Total daily dose |
|---|---|---|
| Day 1 | 5 mg daily. (2.5 mg for high sensitivity) | |
| 2 to 3 days after initiation | <1.5. 1.5-1.9. 2.0-2.5. >2.5. | 5-7.5 mg daily. 2.5-5 mg daily. 2.5 mg daily. Hold warfarin and check INR next day. |
| 2 to 3 days after the last INR check | <1.5. 1.5-1.9. 2.0-3.0. >3 | 7.5-10 mg daily. 5-10 mg daily. 2.5-5 mg daily. Hold warfarin and recheck INR in 1-2 days. |
Frequency of INR monitoring after Initiation of Warfarin
Check INR in the following way:
- Every 2-3 days: Until INR is within therapeutic range on 2 consecutive INR checks. Then,
- Every week: Until INR is within therapeutic range on 2 consecutive INR checks. Then,
- Every 2 weeks: Until INR is within therapeutic range on 2 consecutive INR checks. Then,
- Every 4 weeks: when dose is stable, check monthly.
Maintenance of warfarin
Warfarin maintenance dosing protocol to maintain an INR 2-3 is as follows:
| INR value | Adjustment |
| <1.5 | Extra dose Increase weekly dose by 10% |
| 1.5-1.9 | Increase weekly dose by 5% |
| 2.0-3.0 | No change |
| 3.1-4.0 | Decrease weekly dose by 5% |
| 4.1-5.0 | *Withhold 1 dose. Decrease weekly dose by 10% |
| 5.1-9.0 | Withhold 2 doses. Decrease weekly dose by 20% |
| >9.0 | Admit |
*History and physical examination to exclude bleeding. Admit persons with additional risks for bleeding.
Frequency of INR monitoring for maintenance of warfarin
Check INR:
- Every 3-5 days: If starting/stopping interacting medication, change in diet, change in activity level or other change that could affect INR.
- Every 1-2 weeks: If dose needed adjustment by 5-10%.
- Every 4 weeks: If maintained on stable dose for <6 months.
- Every 6-8 weeks: If maintained on the same stable dose for >6 months
For Indications of Warfarin, check Atrial Fibrillation.