Sarah E. Wilson, MSc, MD, PGY1, Internal Medicine, University of Toronto, Toronto, ON.
Mark A. Crowther, MD, MSc, Associate Professor of Medicine, Department of Medicine, McMaster University, Hamilton, ON.

Warfarin reduces the risk of thrombotic complications in a wide range of patients and appears to be particularly effective in older adults. Warfarin initiation should be undertaken with care in the older adults because they are likely to require smaller maintenance doses to achieve the same target international normalized ratio (INR). Inappropriate prescribing of medications among older adults increases the risk of drug interactions that may alter warfarin anticoagulation. Such interactions should be anticipated and monitored to ensure that over- or under-anticoagulation do not persist. A range of strategies are available to follow warfarin therapy in the outpatient setting to ensure safe and effective anticoagulation.
Key words: warfarin, anticoagulation, vitamin K, atrial fibrillation.