Warfarin is an indirect antagonist of vitamin K which is required for the synthesis of active clotting factors II (prothrombin), VII, IX and X.
These clotting factors are synthesised in the liver and require vitamin K. Vitamin K is required for the carboxylation of specific glutamate residues in the clotting factor precursors. Warfarin acts by inhibiting vitamin K epoxide reductase which converts vitamin K epioxide back to the reduced form.
The half-life of the different clotting factors (II, VII, IX, X) are not the same (II = 60h; VII = 6h; IX = 24h; X = 40h) and so there is a delay before the full effect of any given dose of warfarin is achieved.
The effect of warfarin is dependent on the concentration of warfarin in the liver (dependent on the pharmacokinetic characteristics of the drug) and the rate of accumulation of the drug (dependent on the half-life of warfarin).
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