Medications That Increase INR in Warfarin Users

Introduction to Medication Interactions

The American Heart Association recommends monitoring INR levels when co-prescribing warfarin with antibiotics, NSAIDs, SSRIs, and certain chemotherapeutic agents, as these medications can significantly increase INR through CYP450 enzyme inhibition, protein displacement, or gut flora alteration 1

Antibiotics represent the most clinically significant drug interaction class, with some agents nearly doubling bleeding risk compared to warfarin alone, according to the American College of Cardiology 1
Erythromycin potentiates warfarin effects 3
Sulfonamides nearly double bleeding risk compared to warfarin alone 2
All antibiotics can alter gut microbiome, reducing vitamin K production and potentiating warfarin 2
Many antibiotics inhibit CYP2C9 or CYP3A4 2
Full enzyme induction/inhibition takes 2-4 weeks to develop and persists 2-4 weeks after discontinuation 3, 2

Triazole antifungals pose severe bleeding risk through potent CYP2C9 inhibition, according to the American College of Cardiology 3
Voriconazole inhibits CYP2C9 and moderately inhibits CYP2C19 3
Miconazole (even topical/vaginal) increases INR; nystatin is a safer alternative 3
Ketoconazole and itraconazole are strong CYP3A4 inhibitors requiring close monitoring 3

NSAIDs double bleeding risk through both pharmacokinetic and pharmacodynamic mechanisms, according to the American Heart Association 1
All NSAIDs (non-selective COX and COX-2 selective) double bleeding risk versus warfarin alone through protein displacement and direct antiplatelet effects 1
Acetaminophen causes dose-dependent INR elevation; doses >9.1 g/week increase risk of INR >6 by 10-fold 1
Acetaminophen doses >2 g/day increase INR by average 0.7 by day 3 1

SSRIs more than double bleeding risk through CYP450 inhibition and antiplatelet effects, according to the American College of Cardiology 1, 4
Fluvoxamine and fluoxetine have the highest risk; inhibit both CYP2C9 and CYP3A4 1, 4

Fluvastatin has the highest risk; inhibits CYP2C9 directly affecting potent S-isomer of warfarin, according to the American Heart Association 5
Atorvastatin, rosuvastatin, simvastatin increase mean INR by ~0.3 at 4 weeks through protein displacement and CYP3A4 inhibition 5
Pitavastatin and atorvastatin have the lowest impact on INR per AHA Scientific Statement 5

Fluoropyrimidines require the most aggressive warfarin dose reductions, according to the American College of Cardiology 1
Fluorouracil requires prophylactic 20-70% warfarin dose reduction through CYP2C9 inhibition, GI tract injury, and gut flora alteration 1

Monitor INR more frequently during initiation and discontinuation of any interacting medication, according to the American College of Cardiology 3, 2
Consider preemptive dose reductions for highest-risk agents (metronidazole, fluorouracil, gemfibrozil) 1, 5, 2