Dilantin is a drug with very odd absorption characteristics.
When it first came out, it was arbitrarily made with fillers that Parke Davis chose for their ease of manufacture. When generics came out, none of the companies could duplicate the absorption for an AB approval from the FDA. So for a very long time, Dilantin brand name was the only type used.
Then Mylan succeeded and they called their generic extended release:
Marketed formulations of extended phenytoin sodium -- long a difficult medication to titrate because of its unique absorption and saturable elimination pharmacokinetics -- may not be bioequivalent, according to studies published recently in the journal Neurology. Specifically, bioavailability of Mylan's extended phenytoin sodium was lower when the product was taken with a high-fat meal, but Parke-Davis's Dilantin Kapseals were unaffected by ingestion of a high-fat meal.
Long History of Problems
With a narrow therapeutic range and nonlinear pharmacokinetic characteristics, phenytoin has always presented a challenge to clinicians, pharmacists, pharmacokineticists, and patients. Products contain both the free acid (tablets and suspension) and the sodium salt (capsules and injection), and the amount of active drug delivered by free acid formulations is 9% higher than that delivered by sodium salts.
Since the phenytoin in Dilantin Kapseals is more slowly absorbed than are other formulations, USP and FDA created prompt and extended release phenytoin sodium categories. Currently, the Mylan extended phenytoin sodium product is "AB" rated by FDA, indicating bioequivalence. Zenith markets a prompt phenytoin sodium capsule, and it should not be substituted for extended phenytoin.
The whole article can be accessed if you join Medscape...it is free. We are not allowed to copy whole articles here, I am sorry.
But because of this inherent problem with phenytoin, it is very important you only use ONE brand consistently so your blood levels can be compared to each other.
Also food interacts with this drug:
this article explains it.
Background:— Because of phenytoin’s narrow therapeutic index and nonlinear pharmacokinetics, food-induced alterations in absorption may markedly influence drug concentrations and, in turn, safety and effectiveness. Potential food-associated differences between 100-mg Mylan (Mylan Pharmaceuticals) extended-release phenytoin sodium capsules and Parke–Davis 100-mg Dilantin Kapseals were examined.
If you are using this drug for pain management, the blood levels are not quite as critical as if you were relying on it for seizure control. Also consumption of high dose folate may reduce this drug's effectiveness for preventing seizures.
High dose folate (or consumption of folate enriched foods) may impair blood levels.