Primary drug	Interacting drug	Effect
Increased effect
Azathioprine (Imuran)	Allopurinol (Zyloprim)	Bone marrow suppression
Digoxin	Diuretics, quinidine	Digitalis intoxication
Methotrexate	Aspirin, sulfisoxazole (Gantrisin)	Bone marrow suppression
Phenytoin (Dilantin)	Chloramphenicol (Chloromycetin),* isoniazid (Nydrazid), phenylbutazone (Butazolidin)*	Cerebellar ataxia, nystagmus, sedation
Propranolol (Inderal)	Cimetidine (Tagamet)	Bradycardia
Sedative hypnotics	Ethanol	Excessive sedation
Sulfonylureas	Chloramphenicol, phenylbutazone, sulfaphenazole,* warfarin	Hypoglycemia
Warfarin (Coumadin)	Aspirin, metronidazole (Flagyl), phenylbutazone	Hemorrhage
Decreased effect
Chlorpropamide (Diabinese)	Corticosteroids	Decreased hypoglycemic effects
Lincomycin (Lincocin)	Kaolin-pectin (Kaodene NN)	Decreased bioavailability
Metformin (Glucophage)	Risperidone (Risperdal)	Decreased hypoglycemic effects
Pioglitazone (Actos)	Olanzapine (Zyprexa)	Decreased hypoglycemic effects
Prednisone	Barbiturates	Decreased corticosteroid effect
Quinidine	Barbiturates, rifampin (Rifadin)	Loss of antiarrhythmic effect
Tetracycline	Antacids–iron	Decreased bioavailability
Warfarin	Barbiturates, aminoglutethimide (Cytadren), rifampin	Loss of anticoagulation