Fosphenytoin Dosage
This dosage information may not include all the information needed to use Fosphenytoin safely and effectively. See additional information for Fosphenytoin.
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Usual Adult Dose for:
Additional dosage information:
Usual Adult Dose for Status Epilepticus
Loading dose: 15 to 20 mg of phenytoin sodium equivalents/kg IV at 100 to 150 mg PE/min. Maximum dose 2,000 mg.
Maintenance dose: 4 to 6 mg/kg/day IV.
Because of the risk of hypotension, fosphenytoin should be administered no faster than 150 mg PE/min.
Usual Adult Dose for Epilepsy
Loading dose: 10 to 20 mg of phenytoin sodium equivalents/kg IV at a rate of 100 to 150 mg/min. Maximum dose 2,000 mg.
Maintenance dose: 4 to 6 mg/kg/day IV in 3 to 4 divided doses.
Because of the risk of hypotension, fosphenytoin should be administered no faster than 150 mg PE/min.
Renal Dose Adjustments
Data not available
Liver Dose Adjustments
Data not available
Dose Adjustments
The dose is expressed as phenytoin sodium equivalents. Fosphenytoin may be substituted at the same total daily dose as phenytoin. Oral phenytoin may be used for maintenance therapy after the fosphenytoin loading dose.
Precautions
The Institute for Safe Medication Practices recommends that organizations which stock fosphenytoin check their automated dispensing cabinet (ADC) screen display, shelf labels, and printed requisitions to ensure the total drug content per container is identified instead of the concentration per mL. The information communicated should be consistent with the manufacturer's label (100 mg PE /2 mL, or 500 mg PE/10 mL) to lessen the potential for confusion. Any auxiliary or computer-generated labels that state 50 mg PE/mL should be removed from bins in the ADCs, as practitioners may rely on those labels, instead of the manufacturer's label, to identify the amount in the vial.
Because the 10 mL vial appears to be most frequently involved in massive overdoses, the Institute for Safe Medication Practices recommends that pediatric facilities should consider stocking only the 2 mL vials of fosphenytoin in the ED. The need to retrieve many vials in order to prepare a single dose (i.e., ten 2 mL vials would be needed to prepare a 1,000 mg PE dose versus just two 10 mL vials) may serve to alert practitioners that a dosing error has been made before the error reaches the patient.
Continuous monitoring of the EKG, blood pressure, and respiratory rate is recommended during IV infusion and for 10 to 20 minutes after discontinuation.
The full antiepileptic effect of fosphenytoin is not immediate and other measures, including IV lorazepam or diazepam administration, may be needed to control status epilepticus. The interpretation of total phenytoin levels should be made with caution in patients with renal or hepatic disease. Unbound phenytoin levels may be more useful in these patients.
Fosphenytoin is contraindicated in patients with sinus bradycardia, sinoatrial block, second and third degree AV block, and Adams-Stokes syndrome.
Safety and effectiveness have not been established in pediatric patients (less than 18 years of age).
Dialysis
Data not available
Other Comments
Doses of fosphenytoin are expressed as their phenytoin sodium equivalents. Therefore, no adjustment is recommended when substituting fosphenytoin for phenytoin or phenytoin for fosphenytoin.
Total phenytoin levels should not be monitored until after conversion is complete. Fosphenytoin is converted to phenytoin within 2 to 4 hours after the end of IV and IM administration, respectively. If determination of serum concentration is required prior to conversion, it is recommended that unbound phenytoin levels be measured. Total phenytoin levels generally range between 10 to 20 mcg/mL and unbound levels fall between 1 to 2 mcg/mL.
