<prompt> The anticoagulant action of warfarin is potentiated by all except:PhenylbutazoneVitamin KRifampicin2 3  <response>: A. Enhanced anticoagulant actionBroad-spectrum antibiotics: Inhibit gut flora and reduce Vit K Production.Some cephalosporins ceftriaxone, cefoperazone cause hypoprothrombinaemia by the same machanism as warfarin-produce additive action.Aspirin: Inhibits platelet aggregation and causes g.i. bleeding-this may be hazardous in anticoagulated patients. High doses of salicylates have synergistic hypoprothrombinemic action and also displace warfarin from protein binding site.Long acting sulfonamides, indomethacin, phenytoin and probenecid: displace warfarin from plasma protein binding.Chloramphenicol, erythromycin, celecoxib, cimetidine, allopurinol, amiodarone and metronidazole: inhibit warfarin metabolism.Tolbutamide and phenytoin: Inhibit warfarin metabolism and vice versa.Liquid paraffin habitual use: Reduces vit K absorption.B. Reduced anticogulant actionBarbiturates but not benzodiazepines. Carbamazepine, rifampin and griseofulvin induce the metabolism of warfarin. The dose of anticoagulant determined during therapy with these drugs would be higher: if the same is continued after withdrawing the inducer-marked hypoprothrombinemia can occur-fatal bleeding is on record.Oral contraceptives: Increase blood levels of clotting factors. The important features of heparin and warfarin are compared.Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 670 