Phenytoin
Phenytoin is an anti-epileptic medication primarily used to control generalized tonic-clonic seizures and focal seizures. It works by blocking voltage-dependent sodium channels in neurons, reducing the abnormal electrical activity that causes seizures. This medication requires careful blood level monitoring due to its narrow therapeutic window and numerous potential drug interactions.
Phenytoin is an anti-seizure medication that works by blocking sodium channels in the brain to reduce abnormal electrical activity.
This medication carries a boxed (black box) warning — the strongest type of FDA warning. Review the full prescribing information before use.
Key facts at a glance
- ATC
- N03AB02
- RxNorm
- 8183
- Generic
- Phenytoin
- Class
- atc-n03
- Brands
- Euthasol, Dilantin, Euthaphen, Phenytek
- Availability
- rx
- Lactation
- unknown
- Routes
- oral
- Forms
- injection, solution, suspension, capsule, tablet
- Regions
- US, EU, CN, JP
- Monitoring
- —
This page is for informational purposes only.
Do not use this information to diagnose yourself or to replace a physician's advice. Always consult a qualified healthcare provider before starting, stopping, or changing any medication. If you believe you are experiencing a medical emergency, call your local emergency number immediately.
Dosage and administration
- Usual dose
- 500mg twice daily
- Maximum
- consult physician
- Frequency
- every 12 hours
- Instructions
- Follow physician instructions
- Duration limits
- as prescribed
- Monitoring
- standard
- Usual dose
- 250mg twice daily
- Maximum
- consult physician
- Frequency
- every 12 hours
- Instructions
- Follow physician instructions
- Duration limits
- as prescribed
- Monitoring
- standard
- Usual dose
- 400mg daily
- Maximum
- consult physician
- Frequency
- once daily
- Instructions
- Follow physician instructions
- Duration limits
- as prescribed
- Monitoring
- standard
Overdose information
Special Populations
7 groupspregnancy
first trimesteravoid if possible, use lowest effective dose
avoid use during pregnancy unless benefits to mother outweigh risks to fetus; consider alternative antiepileptic drugs
regular fetal ultrasound monitoring, maternal phenytoin serum levels
Use of this medication in this population requires careful consideration of risks and benefits.
lactation
breastfeedingno adjustment typically required
use with caution; monitor infant for signs of toxicity including sedation, poor feeding, or rash
monitor infant for adverse effects, consider infant serum levels if symptoms occur
Use with caution and monitor for adverse effects.
geriatric
elderlyreduce initial dose, use lower maintenance doses
start at lower doses and titrate slowly; monitor for increased sensitivity to CNS effects
regular phenytoin serum level monitoring, monitor for signs of toxicity
Use with caution and monitor for adverse effects.
pediatric_age
infants and childrenweight-based dosing, slower titration
use weight-based dosing with careful titration; IV form requires slow administration to avoid cardiovascular depression
frequent serum level monitoring especially during growth, monitor for gingival hyperplasia and hirsutism
Use with caution and monitor for adverse effects.
renal_impairment
severe renal impairmentmay require dose reduction in severe impairment
monitor free phenytoin levels; protein binding may be reduced in uremia
monitor free (unbound) phenytoin levels rather than total levels
Use with caution and monitor for adverse effects.
hepatic_impairment
severe hepatic impairmentsignificant dose reduction required
use with extreme caution; reduced hepatic metabolism leads to prolonged half-life and accumulation
frequent serum level monitoring, monitor for signs of toxicity
Use of this medication in this population requires careful consideration of risks and benefits.
pregnancy
third trimesteravoid if possible, use lowest effective dose
monitor maternal and cord blood phenytoin levels at delivery; vitamin K supplementation may be considered
frequent monitoring near delivery, assess for neonatal bleeding risk
Use of this medication in this population requires careful consideration of risks and benefits.
Statistics and clinical data
Drug class
Phenytoin belongs to the hydantoin class of antiepileptic drugs
Common uses
Primary therapeutic indications for phenytoin
FDA approval year
Historical FDA approval date for phenytoin
Mechanism of action
Primary pharmacodynamic mechanism
Phenytoin is used to treat and prevent generalized tonic-clonic seizures, focal (partial) seizures, and status epilepticus. It is not typically used for absence seizures and may worsen certain types of seizures in some patients.
Phenytoin blocks voltage-dependent sodium channels in neuronal membranes, stabilizing neural tissue and reducing the ability of neurons to fire at high frequencies, thereby preventing the spread of seizure activity.
Phenytoin has a narrow therapeutic index requiring regular blood level monitoring. Common side effects include gum overgrowth, dizziness, and drowsiness. It has significant drug interactions and should not be stopped abruptly. Pregnant patients should be aware of potential fetal risks.
Contraindications
Conditions and patient states where this medication should be avoided or used with caution.
Do not use (1)
Known allergy to this medication or its components
Use caution (1)
Consult healthcare provider for benefits vs risks assessment
Official drug label
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This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed July 26, 2026. · Read our editorial policy