Phenytoin Sodium NRIM 100mg Capsules

Phenytoin is licensed for the control of tonic-clonic (grand mal), partial seizures or a combination of these seizure types. Phenytoin can also be used to prevent or treat seizures arising during or following neurosurgery and/or severe head injury, and as a second line treatment option for the management of trigeminal neuralgia.

Epilepsy is one of the most common and serious neurological conditions within the UK. Epilepsy Action estimates that around 600,000 people in the UK are affected by the condition[1].

Numark Information Services have been contacted by several members regarding Phenytoin Sodium NRIM 100mg capsules (Lime Pharma) and whether they can be supplied to patients stabilised on the phenytoin hard capsule manufactured by Flynn Pharma.

The Commission on Human Medicines (CHM)[2] classifies phenytoin as a category 1 anti-epileptic drug; this means that to ensure ongoing seizure control an epileptic patient should be maintained on a specific brand or generic. Consequently patients currently stabilised on the Flynn Pharma generic should not be switched to the NRIM version.

Differences in drug solubility and membrane permeability can lead to formulation differences that affect product bioavailability.

The low solubility and membrane permeability of phenytoin coupled with an elimination rate that is subject to inter-patient variability has led to phenytoin being considered the AED with the highest potential for product variability. Consequently any differences in product formulation could theoretically have a major impact on bioavailability.

In addition, phenytoin has a narrow therapeutic range which means that dose limiting side effects and toxicity can occur at therapeutic drug levels.  A small change in bioavailability could potentially lead to toxicity or loss of seizure control.

Advice for patients and healthcare professionals

Patients should be maintained on the same brand or generic version of phenytoin.

Prescribers are advised to specify the brand required on the prescription to ensure that brand continuity is maintained.

Pharmacists should confirm with patients prescribed phenytoin generically which brand or generic they normally take.Where it is not possible to maintain continuity of supply close collaboration with patient and prescriber is necessary to minimise the risks associated with AED switching.

Patients should be advised to speak to the pharmacist if they believe that the phenytoin product they have been supplied is different to their normal brand.