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 
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