Warfarin Drug–Drug Interactions to Monitor Closely
Warfarin has clinically important interactions that increase INR (bleeding risk) with CYP450 inhibitors, and decrease INR (thromboembolic risk) with CYP450 inducers. INR should be monitored closely after starting, stopping, or changing doses of interacting medications. [1]
CYP2C9 Inhibitors (Common Causes of INR Increase)
CYP2C9 inhibition can increase warfarin exposure and increase INR. [1]
High-yield CYP2C9 inhibitor examples that require INR monitoring:
- Amiodarone (CYP2C9 inhibitor). [1]
- Trimethoprim-sulfamethoxazole (cotrimoxazole) (CYP2C9 inhibitor). [1]
- Fluconazole (CYP2C9 inhibitor). [1]
- Metronidazole (CYP2C9 inhibitor). [1]
CYP3A4 and CYP1A2 Inhibitors (Additional Mechanisms for INR Increase)
Warfarin interactions also occur with inhibitors of CYP3A4 and CYP1A2, which can increase INR. [1]
Examples requiring INR monitoring:
- CYP3A4 inhibitors such as clarithromycin and erythromycin (listed as CYP3A4 inhibitors). [1]
- CYP3A4 inhibitors such as fluconazole are listed under CYP3A4 inhibitors as well. [1]
CYP450 Inducers (Causes of INR Decrease)
CYP450 induction can reduce warfarin exposure and decrease INR. [1]
Common inducer examples that require INR monitoring:
- Rifampin (CYP2C9 inducer listed and also CYP3A4 inducer listed). [1]
- Carbamazepine and phenobarbital (CYP2C9 inducers listed). [1]
Antiplatelet and NSAID Co-Use (Increased Bleeding Risk Beyond INR Changes)
Bleeding risk increases when warfarin is combined with agents that impair hemostasis, including antiplatelet drugs and NSAIDs. [2]
High-priority co-medications to review:
- Aspirin and NSAIDs (augment warfarin’s bleeding risk via platelet inhibition). [2]
Antibiotics and Antifungals With Special INR/bleeding Relevance
Antimicrobials can increase INR through CYP inhibition, but some antibiotics can also alter vitamin K availability via gut flora effects. [3]
Clinically high-yield antibiotics/antifungals to watch:
- Trimethoprim-sulfamethoxazole. [1]
- Metronidazole. [1]
- Fluconazole. [1]
Initiation, Discontinuation, and Monitoring Triggers
INR should be checked after starting, stopping, or changing the dose of a interacting medication due to delayed effects and variability. [1]
More intensive INR monitoring is required after starting high-risk interacting drugs such as amiodarone and TMP-SMX. [3]
Common Pitfalls to Avoid
Empiric warfarin dose adjustment should not be assumed to be unnecessary when interacting drugs are started or stopped because clinically significant INR shifts occur after many medication changes. [1]
INADEQUATE monitoring is a recurring problem during concomitant antibiotic therapy because INR elevation can occur with antimicrobial exposure. [4]