July 2010



Indications, risks and drug interactions

Volume 39, No.7, July 2010 Pages 476-479

Rami Tadros

Sepehr Shakib


Warfarin is a commonly used medication for the prevention and treatment of venous thromboembolism. It can be challenging for both the patient and the prescriber to manage at times.


To describe the mechanism of action of warfarin, and to discuss the indications for warfarinisation, the risks associated with warfarin use, and some of its drug interactions.


The common indications for warfarinisation are atrial fibrillation, venous thromboembolism and prosthetic heart valves. Contraindications include absolute and relative contraindications, and an individualised risk-benefit analyses is required for each patient. There are many interactions with warfarin, including pharmacokinetic and pharmacodynamic. Pharmacokinetic interactions can be monitored by using International Normalised Ratio levels. Pharmacodynamic interactions require knowledge by the prescriber to predict any interactions with warfarin, and International Normalised Ratio monitoring assists.

Warfarin is commonly used in general practice to treat and prevent thrombosis in a range of clinical settings. It acts by antagonising the action of vitamin K resulting in production of defective clotting proteins. It is cleared via the cytochrome P450 enzymes in the liver and is subject to interactions with a large number of drugs. It is extremely important to increase the frequency of International Normalised Ratio (INR) monitoring whenever a drug is started or stopped while a patient is on warfarin.

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