Warfarin therapy is prolonged by all except:

Barbiturates (but not benzodiazepines), carbamazepine, rifampicin 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 causing fatal bleeding. Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 670