Phenytoin

Phenytoin (brand name Dilantin) is a widely used anticonvulsant medication prescribed to prevent and control seizures in patients with epilepsy and related disorders.

It works by stabilizing neuronal membranes and reducing excitability through selective inhibition of voltage-gated sodium channels in the brain, thereby decreasing abnormal electrical activity that leads to seizures.

Because of its nonlinear pharmacokinetics and narrow therapeutic index, phenytoin dosing typically requires careful monitoring of blood levels to optimize treatment and minimize toxicity.

The pharmacogene Gene2Rx covers for phenytoin is CYP2C9, a liver enzyme responsible for metabolizing phenytoin into inactive compounds. Variants in CYP2C9 alter the enzyme's activity and directly affect drug clearance and blood levels.

Phenytoin also has a separate, well-known immune-mediated risk tied to HLA-B*15:02 (Stevens-Johnson syndrome and toxic epidermal necrolysis), particularly in patients of East Asian ancestry. Gene2Rx does not currently test HLA alleles. Patients of relevant ancestry starting phenytoin should ask their prescriber about HLA-B*15:02 testing through a clinical pharmacogenetics lab.

Genetic differences in CYP2C9 define metabolizer phenotypes (poor, intermediate, normal) that directly influence phenytoin blood concentrations and therapeutic risk.

Normal Metabolizer

• Effect on drug levels: Expected to metabolize phenytoin at typical rates, maintaining standard blood concentrations.
• Clinical consequence: Standard seizure control with usual dosing regimens.
• Side effects: Typical risk of dizziness or ataxia; severity and frequency align with general population rates.

Intermediate Metabolizer

• Effect on drug levels: Slower clearance leading to moderately increased blood concentrations.
• Clinical consequence: Slightly higher risk of dose-dependent toxicity; therapeutic levels should be monitored.
• Side effects: Mild to moderate central nervous system side effects (e.g., nystagmus, drowsiness); may occur somewhat more frequently.

Poor Metabolizer

• Effect on drug levels: Significantly reduced clearance causing substantially elevated blood concentrations.
• Clinical consequence: High risk of toxicity; dose reduction necessary to maintain safe levels.
• Side effects: Severe CNS toxicity such as ataxia and cognitive impairment; increased severity and frequency if unadjusted.

Indeterminate/Not Available

• Effect on drug levels: Unknown due to insufficient genetic information.
• Clinical consequence: No specific guidance; follow standard dosing with clinical monitoring.
• Side effects: Uncertain; monitor for any signs of toxicity as per routine practice.

CYP2C9 Dosing Guideline (CPIC)

Phenytoin carries a separate, well-known immune-mediated risk that is not based on the CYP2C9 genetics covered in your Gene2Rx report. The HLA-B*15:02 allele is strongly associated with phenytoin-induced Stevens-Johnson syndrome and toxic epidermal necrolysis, particularly in patients of East Asian ancestry. Gene2Rx does not currently test HLA alleles. If your ancestry includes populations where HLA-B*15:02 is common, ask your prescriber whether HLA-B*15:02 testing is appropriate before starting phenytoin; that testing is offered separately by clinical pharmacogenetic laboratories.

• CPIC, Guideline for Phenytoin and CYP2C9 and HLA-B
• FDA, Table of Pharmacogenetic Associations