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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.

Adverse Effects:

-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

  1. Vegetables and fruits: contains Vitamin K which may affect warfarin action. Consistent vegetable intake is recommended.

  2. Alcohol: may increase INR

  3. 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)

  4. CYP inducers: likewise, inducers will lead to lower warfarin levels resulting in underdosing. Notorious are carbamazepine, phenytoin, phenobarbital.

  5. Antibiotics: may interfere with gut flora, which reduces vitamin K synthesis--> can increase INR

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