Fosphenytoin
Fosphenytoin is an anticonvulsant medication used to treat and prevent certain types of seizures, including status epilepticus. It is a water-soluble prodrug that is converted in the body to phenytoin, allowing for intravenous or intramuscular administration when rapid seizure control is needed. This medication is typically used in hospital settings for patients who cannot take oral antiepileptic drugs.
Fosphenytoin is an injectable anticonvulsant prodrug of phenytoin used to treat and prevent seizures, particularly in emergency and hospital settings.
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
- N03AB05
- RxNorm
- 72236
- Generic
- Fosphenytoin
- Class
- atc-n03
- Brands
- Cerebyx
- Availability
- rx
- Lactation
- unknown
- Routes
- —
- Forms
- injection
- 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
avoid use during pregnancy unless benefits outweigh risks; consider alternative antiseizure medications with better safety profile
regular fetal monitoring including ultrasound for fetal growth
Use of this medication in this population requires careful consideration of risks and benefits.
pregnancy
second third trimestermonitor levels closely
maintain therapeutic phenytoin levels; dose adjustments often needed as pregnancy progresses due to altered pharmacokinetics
frequent phenytoin plasma level monitoring; fetal growth monitoring
Use with caution and monitor for adverse effects.
lactation
noneavoid or discontinue breastfeeding
fosphenytoin and its metabolite phenytoin pass into breast milk; breastfeeding not recommended
monitor infant for signs of phenytoin exposure including sedation, feeding difficulties
Use of this medication in this population requires careful consideration of risks and benefits.
geriatric
nonereduce initial dose
start at lower end of dosing range; reduced hepatic metabolism and protein binding in elderly
monitor phenytoin levels and for signs of toxicity including ataxia, dizziness, nystagmus
Use with caution and monitor for adverse effects.
pediatric_age
neonates infantsweight-based dosing with careful titration
approved for use in pediatric patients including neonates; doses expressed as phenytoin equivalents (PE)
closely monitor phenytoin levels; monitor for cardiac effects especially IV administration
Use with caution and monitor for adverse effects.
renal_impairment
mild to moderatemay require dose reduction
fosphenytoin is metabolized to phenytoin which is highly protein bound; in renal impairment, monitor free phenytoin levels
monitor both total and free phenytoin levels; watch for toxicity
Use with caution and monitor for adverse effects.
hepatic_impairment
mild to moderatesignificant dose reduction required
fosphenytoin is metabolized in liver to phenytoin; hepatic impairment reduces metabolism
frequent phenytoin level monitoring; monitor liver function tests
Use with caution and monitor for adverse effects.
Statistics and clinical data
Commonly prescribed since
FDA approval year for intravenous use
Anticonvulsant
ATC classification N03AB12 - phenytoin derivatives
Common uses
Primary therapeutic indications
Route of administration
Available formulations
Treatment and prevention of generalized tonic-clonic seizures, complex partial seizures, and status epilepticus. Primarily used in acute care settings when rapid IV or IM administration is required.
Fosphenytoin is a prodrug that is converted to phenytoin in the bloodstream. Its anticonvulsant effects are achieved through voltage-dependent blockade of sodium channels in neurons, which stabilizes neuronal membranes and prevents the spread of seizure activity.
Must be administered slowly via IV (no faster than 150 mg PE/minute) due to risk of severe cardiovascular reactions including hypotension and arrhythmias. Requires cardiac monitoring during infusion. May cause drowsiness, dizziness, or coordination problems. Cross-reacts with phenytoin in serum drug testing.
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