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What is The anticoagulant action of warfarin is potentiated by all except: and explain in detail?
A. Enhanced anticoagulant action

Broad-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 action

Barbiturates (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