PEBC: What You Need to Know: Warfarin
Warfarin is a Vitamin K antagonist. Vitamin K is used as a cofactor in the blood clotting cascade.
Indications: deep vein thrombosis, stroke prevention in atrial fibrilation
Dosing: once daily at the same time each day. The dose varies greatly based on genetics, interactions and age. Younger patients require higher doses compared to older patients.
-monitoring of the International Normalized Ratio (INR) is important to reduce bleeding. The target is usually 2 to 3 for most indications (ideally 2.5). High INR may lead to bleeding, whereas low INR is ineffective.
-bruising and cuts may take longer to heal
-severe: melena, hemotypsis
Drug Interactions: TOO MANY
Vegetables and fruits: contains Vitamin K which may affect warfarin action. Consistent vegetable intake is recommended.
Alcohol: may increase INR
CYP inhibitors: careful of 1A2, 2C9 and 3A4 inhibitors which can increase warfarin levels --> sulfamethoxazole (2C9), metronidazole (2C9), clarithromycin (3A4), ciprofloxacin (1A2, 3A4), fluconazole (2C9), amiodarone (1A2, 2C9, 3A4)
CYP inducers: likewise, inducers will lead to lower warfarin levels resulting in underdosing. Notorious are carbamazepine, phenytoin, phenobarbital.
Antibiotics: may interfere with gut flora, which reduces vitamin K synthesis--> can increase INR