Dosage of warfarin

Warfarin’s dosage is individualised for each patient, with the dose adjusted to achieve a target international normalised ratio (INR). Follow a local protocol to determine the starting dose of warfarin. If a local protocol is not available, an example of a validated age-adjusted protocol for starting warfarin is given in Age-adjusted protocol for starting warfarin.

Factors that predict a lower maintenance dose of warfarin include advanced age, severe liver disease, malnourishment, Asian ethnicity, sensitivity to warfarin therapy, and concurrent therapy with drugs that increase the effect of warfarin.

Genetic polymorphisms contribute to the variability of warfarin dosing, but the clinical utility of pharmacogenetic testing is not validated in routine practice. The different brands of warfarin (eg Coumadin, Marevan) have not been shown to be bioequivalent and should not be interchanged.

For the management of patients who are bleeding or have a high INR, see Bleeding and overanticoagulation in patients taking warfarin therapeutically.

Table 1. Age-adjusted protocol for starting warfarin
Printable table
50 years or younger 51 to 65 years 66 to 80 years older than 80 years
50 years or younger
	INR [NB1]	Dose (mg) [NB1] [NB2]
Day 1	less than 1.4	10
Day 2	1.5 or less	10
Day 2	1.6 or more	0.5
Day 3	1.7 or less	10
	1.8 to 2.3	5
	2.4 to 2.7	4
	2.8 to 3.1	3
	3.2 to 3.3	2
	3.4	1.5
	3.5	1
	3.6 to 4.0	0.5
	more than 4	withhold dose
Day 4	1.5 or less	10 to 15
	1.6	8
	1.7 to 1.8	7
	1.9	6
	2.0 to 2.6	5
	2.7 to 3.0	4
	3.1 to 3.5	3.5
	3.6 to 4.0	3
	4.1 to 4.5	omit dose for day 4, then on day 5 use: 1 to 2
	more than 4.5	withhold dose
51 to 65 years
	INR [NB1]	Dose (mg) [NB1] [NB2]
Day 1	less than 1.4	9
Day 2	1.5 or less	9
Day 2	1.6 or more	0.5
Day 3	1.7 or less	9
	1.8 to 2.3	4.5
	2.4 to 2.7	3.5
	2.8 to 3.1	2.5
	3.2 to 3.3	2
	3.4	1.5
	3.5	1
	3.6 to 4.0	0.5
	more than 4	withhold dose
Day 4	1.5 or less	9 to 14
	1.6	7
	1.7 to 1.8	6
	1.9	5
	2.0 to 2.6	4.5
	2.7 to 3.0	3.5
	3.1 to 3.5	3
	3.6 to 4.0	2.5
	4.1 to 4.5	omit dose for day 4, then on day 5 use: 0.5 to 1.5
	more than 4.5	withhold dose
66 to 80 years
	INR [NB1]	Dose (mg) [NB1] [NB2]
Day 1	less than 1.4	7.5
Day 2	1.5 or less	7.5
Day 2	1.6 or more	0.5
Day 3	1.7 or less	7.5
	1.8 to 2.3	4
	2.4 to 2.7	3
	2.8 to 3.1	2
	3.2 to 3.3	1.5
	3.4	1
	3.5	1
	3.6 to 4.0	0.5
	more than 4	withhold dose
Day 4	1.5 or less	7.5 to 11
	1.6	6
	1.7 to 1.8	5
	1.9	4.5
	2.0 to 2.6	4
	2.7 to 3.0	3
	3.1 to 3.5	2.5
	3.6 to 4.0	2
	4.1 to 4.5	omit dose for day 4, then on day 5 use: 0.5 to 1.5
	more than 4.5	withhold dose
older than 80 years
	INR [NB1]	Dose (mg) [NB1] [NB2]
Day 1	less than 1.4	6
Day 2	1.5 or less	6
Day 2	1.6 or more	0.5
Day 3	1.7 or less	6
	1.8 to 2.3	3
	2.4 to 2.7	2
	2.8 to 3.1	1
	3.2 to 3.3	1
	3.4	1
	3.5	0.5
	3.6 to 4.0	0.5
	more than 4	withhold dose
Day 4	1.5 or less	6 to 9
	1.6	5
	1.7 to 1.8	4
	1.9	3.5
	2.0 to 2.6	3
	2.7 to 3.0	2.5
	3.1 to 3.5	2
	3.6 to 4.0	1.5
	4.1 to 4.5	omit dose for day 4, then on day 5 use: 0.5 to 1
	more than 4.5	withhold dose
Note: INR = international normalised ratio NB1: Patients should take warfarin doses at a consistent time each evening. Take blood samples for INR the following morning. NB2: Consider giving a lower starting dose depending on the patient’s comorbidities and/or concurrent medication. Adapted with permission from Tideman PA, Tirimacco R, St John A, Roberts GW. How to manage warfarin therapy. Aust Prescr 2015;38(2):44-8. doi: 10.18773/austprescr.2015.016