The anticoagulant effects of
warfarin can be markedly reduced (by around 50% in many reports) by
carbamazepine. Acenocoumarol is expected to interact similarly, and cases of this interaction have been seen with phenprocoumon.
Monitor the INR if
carbamazepine is added to
warfarin therapy. Continue monitoring until the
carbamazepine dose is stabilised. Also monitor the INR if the
carbamazepine is withdrawn as the effects of
warfarin may become excessive within a week.