All of the following drugs increase bleeding when given to a patient on warfarin EXCEPT:: (A) Isoniazid, (B) Amiodarone, (C) Carbamazepine, (D) Cimetidine
Answer is C. Ans. C. CarbamazepineWarfarin constitutes S as well as R enantiomers. S Warfarin is the most active form:* Metabolism of S warfarin is by CYP2C9 leading to the formation of 7-OH warfarin* Metabolism of R warfarin is by CYP1A1, CYP1A2, CYP3A4 leading to the formation of 6-OH warfarin, 8-OH warfarin, 10-OH warfarinAs Carbamazepine is enzyme inducer, it induces metabolism of warfarin and hence warfarin concentration decreases and it can lead to therapeutic failureRest of drugs: Isoniazid, Amiodarone, cimetidine are microsomal enzyme inhibitors leading to warfarin toxicity and can lead to bleeding.