The following factors, alone or in combination, may be responsible for DECREASED PT/INR response:

ENDOGENOUS FACTORS:

edema hypothyroidism
hereditary coumarin resistance nephrotic syndrome
hyperlipemia 

EXOGENOUS FACTORS:

Potential drug interactions with warfarin sodium are listed below by drug class and by specific drugs.

Classes of Drugs
Adrenal Cortical Steroid Inhibitors Antithyroid Drugs* Immunosuppressives
Antacids Barbiturates Oral Contraceptives,
Antianxiety Agents Diuretics* Estrogen Containing
Antiarrhythmics* Enteral Nutritional Supplements Selective Estrogen Receptor
Antibiotics* Fungal Medications, Systemic* Modulators
Anticonvulsants* Gastric Acidity and Steroids, Adrenocortical*
Antidepressants* Peptic Ulcer Agents* Tuberculosis Agents*
Antihistamines Hypnotics* Vitamins*
Antineoplastics* Hypolipidemics* 
Antipsychotic Bile Acid-Binding Resins* 
Medications HMG-CoA Reductase Inhibitors* 
*
Increased and decreased PT/INR responses have been reported.
Specific Drugs Reported
alcohol* cyclophosphamide* pravastatin*
aminoglutethimide dicloxacillin prednisone*
amobarbital ethchlorvynol primidone
atorvastatin* glutethimide propylthiouracil*
azathioprine griseofulvin raloxifene
butabarbital haloperidol ranitidine*
butalbital meprobamate rifampin
carbamazepine 6-mercaptopurine secobarbital
chloral hydrate* methimazole* spironolactone
chlordiazepoxide moricizine hydrochloride* sucralfate
chlorthalidone nafcillin trazodone
cholestyramine* paraldehyde vitamin C (high dose)
clozapine pentobarbital vitamin K
corticotropin phenobarbital warfarin sodium underdosage
cortisone phenytoin* 
also:diet high in vitamin K
unreliable PT/INR determinations

Because a patient may be exposed to a combination of the above factors, the net effect of warfarin sodium on PT/INR response may be unpredictable. More frequent PT/ INR monitoring is therefore advisable. Medications of unknown interaction with coumarins are best regarded with caution. When these medications are started or stopped, more frequent PT/INR monitoring is advisable.

It has been reported that concomitant administration of warfarin and ticlopidine may be associated with cholestatic hepatitis.

Botanical (Herbal) Medicines
Caution should be exercised when botanical medicines (botanicals) are taken concomitantly with warfarin sodium. Few adequate, well controlled studies exist eva luating the potential for metabolic and/or pharmacologic interactions betwe