Phenytoin is a commonly prescribed antiepileptic drug in medical practice. As it is a drug which has a non-linear saturable pharmacokinetics, a relatively narrow therapeutic range and a large number of reported pharmacokinetic interactions, plasma phenytoin concentration is inevitable to monitor. Our aim is to emphasize, one of serious interactions of phenytoin with enteral feeding formulas administered together with phenytoin by nasogastric tube. This interaction decreases bioavailability of phenytoin resulting often in detection of subtherapeutic concentrations, though recommended daily doses (recommended daily dose 5–7 mg/kg/day, RDD 300 mg) are administered. In our article, we refer the case of an elderly man admitted to the intensive care unit. Patient’s phenytoin serum concentrations dramatically decreased during simultaneous administration of phenytoin with enteral feeding formulas (Nutrison multi fibre sol.). The exact mechanisms underlying this interaction remain unknown and there are several possible reasons. Careful monitoring of plasma levels is essential for rational and successful administration of phenytoin, especially, if the patients are changing tube-feeding regimen or changing from the parenteral to the enteral dosage form of phenytoin.

        Key words: phenytoin – monitoring – enteral feeding – interaction