 This is often explained as being due to the effect of phenylbutazone ejecting warfarin from its plasma protein, thus increasing the concentration of free warfarin and increasing its anticoagulant effect.However, the real problem is that phenylbutazone interferes with the liver's ability to metabolize warfarin so free warfarin cannot be metabolized properly or excreted.This leads to an increase in free warfarin and the resulting bleeding issues.