Table 1.

Suggested management strategies for warfarin pharmacokinetic drug interactions: S-warfarin (CYP2C9/CYP3A4) and R-warfarin (CYP1A2/CYP3A4/CYP2C19)⁷

Inducers26-32	Inhibitors18-21,24,32,33
Monitor the INR within at least 5 d of inducer initiation and then at least once to twice weekly	Monitor the INR 3-5 d after inhibitor initiation (after 1 wk with amiodarone) and adjust warfarin dose accordingly
Consider aggressive warfarin dose increases until therapeutic INR is reached	Expect that patients may need a 20%-50% decrease in warfarin dose from baseline
Expect that patients may need at least a 50%-100% increase in warfarin dose from baseline	Consider other factors that may independently contribute to elevated INR, such as acute infection and dietary Vitamin K changes
Consider seeking an alternative noninteracting drug	Consider seeking an alternative noninteracting drug
Monitor the INR within at least 5 d of inducer discontinuation and then at least once to twice weekly	Allow 3-5 d for inhibitor offset (longer for amiodarone)
Expect to decrease warfarin dose to approximately preinducer levels	Expect to decrease warfarin dose to approximately preinhibitor levels