Acenocoumarol is a short-lived oral anti-coagulant, which, like warfarin, functions by inhibiting vitamin K epoxide reductase. It has higher intrinsic anticoagulant potency than warfarin and phenprocoumon, when evaluated in vitro. Acenocoumarol has a single chiral center that gives rise to two different enantiomeric forms. (R)-(+)-Acenocoumarol has a longer plasma elimination half-life (6.6 hours) and slower plasma clearance (1.9 L/hour), compared to the (S)-(-)-enantiomer (1.8 hours, 28.5 L/hour). The R-enantiomer is rapidly absorbed from the gastrointestinal tract with essentially complete oral bioavailability, whereas (S)-(-)-acenocoumarol undergoes extensive first-pass metabolism. Perhaps related to these pharmacokinetic characteristics, (R)-(+)-acenocoumarol is more potent in vivo as an anti-coagulant than the (S)-(-)-enantiomer. As the clearance of acenocoumarol is ~20-fold faster than that for warfarin, the plasma concentrations of acenocoumarol are substantially lower than those for warfarin in patients receiving long-term treatment.