| 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 |