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Research Center

Medicine Substitutes: Switching to Generic Dialantin



Please tell me what you can about generic dialantin. My husband has successfully taken dialantin for over 30 years to control his grand mal seizures along with phenobarbitol. Our insurance has just made dilantin way more costly. Our pharmacist suggest that we ask our doctor to prescribe the generic and it would be over 30 dollars a month cheaper. It scares us because the seizures are so violent and if he has a seizure he can`t drive. Our 19 year old son is in the same boat.


Dilantin is a medicine used for the management of generalized tonic-clonic seizures (grand mal). Another name for Dilantin is phenytoin sodium extended. Recently an AB rated generic version of Dilantin was approved by the US Food and Drug Administration (FDA).

In order to be approved as generic equivalent (or AB-rated) product by the FDA, the product must have the same dose of the drug, the same route of administration (oral, IV, etc), and the same dosage form as the brand name. Prior to FDA approval, the manufacturer must also provide data from clinical trials showing that the generic drug is bioequivalent to the brand name product. Bioequivalence means that the blood levels achieved by a generic medicine compared to a brand name medicine are similar throughout the entire dosing interval.

It is very important for a supplier of a generic drug to receive an AB rating for the FDA for its product because this rating allows the generic version to be freely substituted for the brand name product. In fact, most states require that pharmacists fill prescriptions written for brand name products with the least expensive AB rated generic product in stock. A consequence of the generic dispensing law is that patients could conceivably get a different (although AB rated) drug product every time they get their prescription refilled. Physicians who want their patient to receive the brand name product can write Do Not Substitute on the prescription. For most medicines, generic equivalent (AB rated) medicines provide the same therapeutic benefit as the brand name product at a significant cost savings for whomever is paying for the medicine This does not seem to be the case with Dilantin and its AB rated generic equivalent.

Clinical trials showed that Dilantin and its generic equivalent were bioequivalent when taken on an empty stomach, and these studies were sufficient for the manufacturer to obtain the AB rating for their product from the FDA. As a result, many third party payors including some state medicaid providers began automatically switching to the generic drug. Subsequently, a number of patients reported loss of seizure control. Additional studies showed that taking the generic version with food reduced its absorption by about 13%. Food does not affect the brand name product to the same extent. This means that a change to the generic product could produce lower blood levels.

The difference in absorption with food between the two products is small, but it turns out that phenytoin is one of a handful of medicines where small changes in the amount of the drug absorbed can result in big changes in serum concentrations. It should be clear that reduced absorption can lead to loss of seizure control. It is also true that slight increases in absorption could result in toxic serum levels over time. Either high or low serum levels could result in hospitalization and lost productivity with related increased costs of care.

It is our opinion that the currently available phenytoin sodium extended products are not truly interchangeable. In general, it seems most appropriate to keep patients on the particular phenytoin sodium extended product on which they have been stabilized. It might be appropriate to start new patients on generic phenytoin sodium extended as long as they can be kept on the same product (no switching) once they are stabilized on it. Dilantin is not a particularly expensive medicine, so switching to a generic is not likely to result in big cost savings. Since it is clear that switching can lead to significantly different serum levels and resultant loss of seizure control or toxicity, it would be prudent to monitor blood levels whenever a switch is made. When the cost of additional serum level monitoring is considered, frequent switching would most likely be cost prohibitive.

Finally, when the risks associated with loss of seizure control or drug toxicity and the associated costs are factored in, switching a stabilized patient from one product to another may not be justifiable. Please note that even when a doctor specifies that a prescription be dispensed as written, third party payers may require that the patient pay a higher co-pay for a brand name product or pay the difference in cost between the generic and brand name versions. Presumably the third party payor would also pay for any additional costs associated with the loss of seizure control or toxicity.

It is important to continue taking your phenytoin sodium extended in a consistent fashion as prescribed. Despite the FDA's AB rating on the generic version of Dilantin, switching from one formulation to another is probably not advisable for most patients. A physician is in the best position to determine which drug and dose is best for a particular patient. The doctor can also monitor blood levels to ensure adequate therapy. Talk to your doctor about any changes in your drug therapy.

This response was prepared in part by Lauren Steffen, a Pharm.D candidate at the University of Cincinnati College of Pharmacy.

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Response by:

Robert James Goetz, PharmD, DABAT
Assistant Professor of Pharmacy Practice
College of Medicine
University of Cincinnati