5 known interactions • 3 major • 2 moderate • 0 minor
Always disclose all medications to your healthcare providers — prescription medicines, OTC medications, vitamins, and herbal supplements. This list may not include every possible interaction. Use our Medicine Interaction Checker to screen your complete medication list.
Potentially life-threatening or causing permanent damage. Avoid combination.
May worsen condition or require dose adjustment. Monitor closely.
Usually limited clinical effect. Manage with routine monitoring.
Virtually all medications (check INR when adding/stopping ANY medicine)
Warfarin has more known medicine interactions than virtually any other medicine. CYP2C9 inhibitors increase INR; CYP2C9 inducers decrease INR; medications affecting platelet function increase bleeding risk.
Management: Check INR within 1 week of starting or stopping any new medication. Educate patients thoroughly.
Antibiotics (fluoroquinolones, metronidazole, fluconazole, macrolides)
Most antibiotics increase INR via gut flora suppression (reducing vitamin K production) or CYP2C9 inhibition.
Management: Monitor INR within 3–5 days of starting antibiotics; dose adjust as needed.
NSAIDs / Aspirin
NSAIDs inhibit platelet function AND can cause GI bleeding, dramatically increasing hemorrhage risk.
Management: Avoid concurrent use. Use acetaminophen for pain. If NSAID required, monitor very closely.
Vitamin K (foods: spinach, kale, broccoli, Brussels sprouts)
High vitamin K intake reduces anticoagulant effect. Consistent (not zero) vitamin K intake is recommended.
Management: Counsel patients to maintain consistent vitamin K intake, not to drastically change diet.
Alcohol
Heavy acute alcohol intake inhibits warfarin metabolism (increased INR). Chronic heavy use induces CYP2C9 (decreased INR). Even moderate use unpredictably affects INR.
Management: Advise consistent moderate alcohol intake or abstinence; monitor INR.
Always ask your pharmacist about potential interactions with food, alcohol, and supplements specific to Warfarin. Some medicines have significant interactions with grapefruit juice, high-fat meals, dairy products, or vitamin K-rich foods.