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Warfarin is an effective and inexpensive anticoagulant, but inappropriate dosing causes serious adverse effects. Whereas roughly half of all patients who receive a fixed dose of warfarin (5 mg daily) have prothrombin times in the therapeutic range (international normalized ratio [INR], 2–3), this dose is too high or too low for many others and can result in fatal bleeding or thrombosis. To improve dosing accuracy, adjustments are often made for clinical variables that affect warfarin’s action and metabolism, such as age, weight, race, and concomitant medications. However, recent findings have shown that one third of the variation in ability to tolerate warfarin is a consequence of mutations in two genes, cytochrome P450 (CYP2C9) and vitamin…