Drug Interactions with Warfarin
Despite its effectiveness, treatment with warfarin can be complicated as many commonly used medications interact. However, interaction doesn’t always mean incompatibility: Often the interaction is predictable and the adjustment to the warfarin dose can be anticipated. At other times, the influence on the INR is more variable and the INR will need to be monitored more frequently so that adjustments can be made to keep the INR in range.
Always make sure each of your doctors knows about all of your medicines from other doctors and double check every medicine change with your anticoagulation provider.
The following is a list of more commonly seen interactions and the potential influence on the INR:
|Drug/Drug Class||Effect on INR||Recommendations/Comments|
|Acetaminophen/APAP||↑ with higher doses||Limit APAP to 2000mg/day|
|Alcohol||↑ with binging
↓ with chronic use
|Limit to 1-2 alcoholic drinks/day|
|Amiodarone||slow ↑ over time||Complex interaction that takes place over 6-8 weeks. Ultimately expect 25-50% warfarin dose reduction. Check INR every week and make adjustments per INR|
|Capecitabine (Xeloda)||↑||>50% warfarin dose reduction possible|
|Dicloxacillin||↓||More significant if course >14 days|
|Doxycycline||↑||Not always clinically significant—check INR more frequently|
|Flu Vaccine||↑||Varies from year to year, but may cause transient rise in INR|
|Fluconazole||↑||Hold warfarin dose once for single dose. Expect 25-50% warfarin dose reduction for extended course|
|Fluoroquinolones||↑||Not always clinically significant—check INR more frequently|
|Metronidazole (Flagyl)||↑||Empiric 25-50% warfarin dose reduction|
|Phenytoin||↓ or ↑||Complex interaction: initially ↑ INR, but then ↓ after prolonged administration. Additionally, warfarin may alter serum phenytoin concentrations and both need to be monitored frequently|
|Rifampin||↓||Expect 2- to 5-fold increase in warfarin dose requirements if course is >2 weeks|
|Sulfamethoxazole (Septra/Bactrim)||↑||Empiric 25-50% warfarin dose reduction|
These are all general guidelines and possible drug interaction situations with warfarin. All dosage adjustments should be made by your physician or pharmacist that is managing your warfarin therapy. The best practice while on warfarin therapy is weekly home INR testing in order to catch any INR rises or falls ahead of time as your medications, diet, or life changes.