Isoniazid can raise the level of phenytoin in your blood. Fosphenytoin turns into phenytoin in the body, so the same applies to Fosphenytoin Sodium. Isoniazid is not the only medicine that does this. The full list of medicines that may raise phenytoin levels is: acute alcohol intake, amiodarone, chloramphenicol, chlordiazepoxide, cimetidine, diazepam, dicumarol, disulfiram, estrogens, ethosuximide, fluoxetine, H2-antagonists, halothane, isoniazid, methylphenidate, phenothiazines, phenylbutazone, salicylates, succinimides, sulfonamides, tolbutamide and trazodone.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.