Here is a short list of medications that will actually prevent a patient from being anticoagulated by coumadin. These medications will make it difficult for the patient to reach therapeutic levels and need to be warned about this drug-drug interaction with coumadin:
Antacids
Antihistamines
Barbituates
Carbamazepine
Cholestyramine
Corticosteroids
Griseofulvin
OCPs
Phenytoin
Rifampin
Vitamin K
Reference: Goldfrank's Textbook of Toxicologic Emergencies, 6th Edition
