Warfarin Drug Interactions Requiring INR Surveillance
Warfarin has clinically important drug–drug interactions that typically occur via CYP450 inhibition/induction, altered vitamin K availability, reduced drug clearance, or additive bleeding risk. [1]
Concomitant interacting drugs should prompt close INR monitoring, particularly when starting or stopping therapy. [1]
Interaction Categories That Most Often Matter
CYP450 inhibitors (INR increase; bleeding risk increase)
- CYP2C9 inhibitors (examples: amiodarone, cotrimoxazole, fluconazole, metronidazole, voriconazole) commonly increase warfarin effect and require close INR monitoring. [2]
- CYP3A4 inhibitors (examples: erythromycin, clarithromycin, fluconazole, voriconazole) can increase warfarin exposure and increase bleeding risk. [2]
CYP450 inducers (INR decrease; thrombosis risk increase)
- CYP2C9 inducers (examples: carbamazepine, phenobarbital, rifampin) can reduce warfarin effect and lower INR. [2]
Additive bleeding-risk combinations (INR may not fully predict risk)
- Drugs that increase bleeding risk (including other anticoagulants or antiplatelet agents) warrant close monitoring because bleeding risk may rise even without proportional INR changes. [2]
High-Yield Drug Classes to Watch
Antibiotics and antifungals
- Trimethoprim-sulfamethoxazole (TMP-SMX) is associated with increased warfarin effect and often requires substantial dose reduction and/or close INR surveillance in practice. UC San Diego Anticoagulation Clinic
- Metronidazole is associated with increased warfarin effect and often requires substantial dose reduction and/or close INR surveillance in practice. UC San Diego Anticoagulation Clinic
- A warfarin–antibiotic study reported that risk of INR ≥ 5.0 was greatest among antibiotics interfering with warfarin metabolism. ASH “Analysis of warfarin drug–drug interactions with antibiotics”
- The warfarin label recommends close INR monitoring when initiating or stopping antibiotics or antifungals. [3]
Antiarrythmics and seizure medications
- Amiodarone is listed as a CYP2C9 inhibitor with potential to increase warfarin effect. [2]
- Carbamazepine and rifampin are listed as inducers with potential to decrease warfarin effect. [2]
Corticosteroids and other short-term agents
- Close INR monitoring is recommended when starting or stopping medications, including short-term drugs such as antibiotics/antifungals and other therapies that may change warfarin exposure. [3]
Medication-Specific Monitoring Timing
- Interaction effects from medications that affect warfarin exposure often begin within 3–5 days after starting interacting therapy. UW Health (Warfarin management inpatient consensus guideline, PDF)
- INR should be checked about 3–4 days after starting a medication with interaction potential. UW Health (Warfarin management inpatient consensus guideline, PDF)
- INR monitoring should also occur when the interacting medication is stopped, since the direction of INR change can reverse. [3]
Practical “Watch List” by Interaction Direction
Likely INR increase (bleeding risk increase)
- TMP-SMX UC San Diego Anticoagulation Clinic
- Metronidazole UC San Diego Anticoagulation Clinic
- Fluconazole, voriconazole [2]
Likely INR decrease (thrombosis risk increase)
- Rifampin [2]
- Carbamazepine and phenobarbital [2]
Common Pitfalls to Avoid
- Assuming INR will remain stable despite antibiotic or antifungal starts. [3]
- Inadequate timing of INR checks after starting interacting drugs. UW Health (Warfarin management inpatient consensus guideline, PDF)
- Failing to account for additive bleeding-risk drugs, including other antithrombotics. [2]
Targeted Output for Clinical Use (INR Surveillance Rule)
- INR should be monitored more frequently when starting, stopping, or changing doses of interacting medications, including antibiotics and antifungals, and when botanical products are changed. [3]