Warfarin Dosing Tonight for Patient on Norco
Resume warfarin tonight at her usual maintenance dose unless her INR is currently supratherapeutic or she has active bleeding. 1
Key Consideration: Norco Does Not Interact with Warfarin
Hydrocodone (the opioid component of Norco) does not interact with warfarin and does not affect INR or anticoagulation. 1
Acetaminophen (the other component of Norco) can significantly elevate INR when taken at doses ≥2 grams/day for several consecutive days, but this interaction requires daily use for 3-7 days before INR elevation becomes clinically apparent. 2, 3, 4
A single dose or sporadic use of acetaminophen does not acutely alter tonight's warfarin dose requirement. 2, 4
Immediate Dosing Decision Algorithm
If her most recent INR is within therapeutic range (2.0-3.0):
- Give her usual maintenance dose of warfarin tonight. 1
- No dose adjustment is needed based solely on Norco use. 1
If her INR is 3.1-3.5:
- Continue her usual dose without modification and recheck INR in 1-2 weeks. 5
- Clinically significant bleeding risk does not rise until INR exceeds 3.5. 5
If her INR is 3.6-5.0:
- Withhold warfarin for 1-2 doses, then restart at a dose reduced by 10-20% of her weekly total. 5
- Do not give vitamin K at this level unless she has high bleeding risk factors (age >65-75 years, prior bleeding, concurrent antiplatelet drugs). 5
If her INR is 5.0-9.0 without bleeding:
- Withhold warfarin for 1-2 doses and monitor serial INR. 5
- Add oral vitamin K 1-2.5 mg only if she has bleeding risk factors. 5
If her INR is >10 without bleeding:
Critical Monitoring Plan for Acetaminophen-Warfarin Interaction
If she will be taking Norco regularly (≥2 grams acetaminophen/day), increase INR monitoring frequency to twice weekly for the first 2-3 weeks. 2, 3
In a prospective randomized trial, 54% of patients taking acetaminophen 2-4 grams/day developed INR elevation ≥0.3 above their therapeutic range by week 2-3. 2
The interaction is dose-dependent: acetaminophen 2 grams/day caused significant INR elevation at week 2, while 4 grams/day caused elevation by week 1. 2
The mechanism is pharmacodynamic (enhanced vitamin K antagonism), not pharmacokinetic, as warfarin plasma levels do not increase. 3
Common Pitfalls to Avoid
Do not preemptively reduce warfarin dose based on anticipated acetaminophen use—wait for actual INR changes with close monitoring. 2, 4
Do not assume sporadic or low-dose acetaminophen (<2 grams/day) will cause problems—the interaction requires sustained daily use at higher doses. 2, 4
Do not delay pain management out of excessive concern about this interaction—it is manageable with appropriate INR monitoring. 2, 4
If she is elderly (>65 years), she likely requires lower warfarin maintenance doses (<5 mg/day) and is more sensitive to both warfarin and the acetaminophen interaction. 5