Most antiepileptic drugs (AEDs) have a narrow therapeutic index, and  have a high potential for CNS-related adverse events, which  is usually related to the serum concentration of the drug. There is no scientific evidence, as of now  to predict that the 20% variability in bioavailabilty recommended by  guidelines can be tolerated by patients with epilepsy. Dosage adjustment is required to provide optimal seizure control while avoiding adverse drug effects. The titration of dose of antiepileptic drugs is guided by target dose, plasma drug concentration reference range or seizure response. In the individual patient the range of effective and tolerable antiepileptic drug dose or plasma drug concentration is often not known. Management regimens may become very complex, with titration sometimes taking weeks in order to avoid adverse effects. Patients need their medication to be consistent during titration so that prescribed changes of dose have predictable consequences.

 Phenytoin is poorly soluble in physiological fluids, and its bioavailability is therefore strongly influenced by various factors, particularly particle size and the substances used as excipients in the formulation.  In designing formulations, there are three pharmacologic risk factors- low water solubility, a narrow therapeutic index and non-linear pharmacokinetics.  Generic drugs contain the identical active ingredient(s) as the innovator medicine drug however the excipients (inactive ingredients) may vary. The rate and extent of absorption or bioavailability often differs between different generic versions of branded products, and each differs from the branded formulation itself, which  can have serious effects for that patient. For example, a slight increase in phenytoin bioavailability can lead to a marked increase in serum level and thus to adverse effects, especially when the level is more than 15mg/L.  Furthermore, nonlinear pharmacokinetics (e.g., phenytoin, PHT) and protein binding (e.g., valproate, VPA) can amplify even slight differences in bioavailability