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Warfarin is the drug of choice; few side effects, well tolerated. A vitamin K antagonist, it takes ~72h to be effective; stable state takes 5-7d. t^ ~35h. Circulates mainly bound to albumin; free warfarin is active. Many drugs t warfarin effect by displacing it from albumin. Monitored by PT using the international normalised ratio (INR).

Administration

Given daily. Usually given with heparin on day 1. If massive thrombosis, delay warfarin for 2-3d. Standard adult regimen = 10mg/d for 2d. Load with caution using reduced dose if liver disease, interacting drugs, patient >80 years. Check INR <1.4 before loading. Check INR daily for first 4 days (see Appendix II, Guidelines on oral anticoagulation: third edition, Brit J Haematol 1998, 101, 374-378) on Day 3, ~16h after second dose, and adjust as follows.

Target INR usually 2.5 except for mechanical heart valves in mitral position when target is 3.0 or 3.5.

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