Formulary
Fosphenytoin sodium: Indications, Dosing, Side Effects and Interactions
Fosphenytoin is a pro-drug of phenytoin.
MedNext Academy | 5 min read
Fosphenytoin sodium: Indications, Dosing, Side Effects and Interactions
Fosphenytoin is a pro-drug of phenytoin.
Drug action
Fosphenytoin is a pro-drug of phenytoin.
Indications and dose
**Status epilepticus**
- **Adult** (By Intravenous Infusion): Initially 20 mg(PE)/kg, dose to be administered at a rate of 100-150 mg(PE)/minute, then 4-5 mg(PE)/kg daily in 1-2 divided doses, dose to be administered at a rate of 50-100 mg(PE)/minute, dose to be adjusted according to response and trough plasma-phenytoin concentration.
- **Elderly** (By Intravenous Infusion): Consider 10-25% reduction in dose or infusion rate.
**Prophylaxis or treatment of seizures associated with neurosurgery or head injury**
- **Adult** (By Intramuscular Injection, Or By Intravenous Infusion): Initially 10-15 mg(PE)/kg, intravenous infusion to be administered at a rate of 50-100 mg(PE)/minute, then 4-5 mg(PE)/kg daily in 1-2 divided doses, intravenous infusion to be administered at a rate of 50-100 mg(PE)/minute, dose to be adjusted according to response and trough plasma-phenytoin concentration.
- **Elderly** (By Intramuscular Injection, Or By Intravenous Infusion): Consider 10-25% reduction in dose or infusion rate.
**Temporary substitution**
- **Adult** (By Intramuscular Injection, Or By Intravenous Infusion): for oral phenytoin Same dose and same dosing frequency as oral phenytoin therapy, intravenous infusion to be administered at a rate of 50-100 mg(PE)/minute.
- **Elderly** (By Intramuscular Injection, Or By Intravenous Infusion): Consider 10-25% reduction in dose or infusion rate.
**Status epilepticus for fosphenytoin sodium**
- **Child 5-17 years** (By intravenous infusion): Initially 20 mg(PE)/kg, dose to be administered at a rate of 2-3 mg(PE)/kg/minute, maximum 150 mg(PE)/minute, then 4-5 mg(PE)/kg daily in 1-4 divided doses, dose to be administered at a rate of 1-2 mg(PE)/kg/minute, maximum 100 mg(PE)/minute, dose to be adjusted according to response and trough plasma-phenytoin concentration.
**Prophylaxis or treatment of seizures associated with neurosurgery or head injury for fosphenytoin sodium**
- **Child 5-17 years** (By intravenous infusion): Initially 10-15 mg(PE)/kg, then 4-5 mg(PE)/kg daily in 1-4 divided doses, dose to be administered at a rate of 1-2 mg(PE)/kg/minute, maximum 100 mg(PE)/minute, dose to be adjusted according to response and trough plasma-phenytoin concentration.
**Temporary substitution for oral phenytoin for fosphenytoin sodium**
- **Child 5-17 years** (By intravenous infusion): Same dose and same dosing frequency as oral phenytoin therapy, intravenous infusion to be administered at a rate of 1-2 mg(PE)/kg/minute, maximum 100 mg(PE)/minute.
Cautions
Heart failure; hypotension; injection solutions alkaline (irritant to tissues); respiratory depression; resuscitation facilities must be available
Contraindications
Acute porphyrias ; second-degree heart block; sino-atrial block; sinus bradycardia; Stokes-Adams syndrome; third-degree heart block
Side effects
Common or very common Asthenia; chills; dizziness; drowsiness; dry mouth; dysarthria; euphoric mood; headache; hypotension; movement disorders; nausea; nystagmus; sensation abnormal; skin reactions; stupor; taste altered; tinnitus; tremor; vasodilation; vertigo; vision disorders; vomiting Uncommon Cardiac arrest; confusion; hearing impairment; muscle complaints; muscle weakness; nervousness; oral disorders; reflexes abnormal; severe cutaneous adverse reactions (SCARs); systemic lupus erythematosus (SLE); thinking abnormal Frequency not known Acute psychosis; agranulocytosis; appetite disorder; atrial conduction depression (more common if injection too rapid); atrioventricular block; bone disorders; bone fracture; bone marrow disorders; bradycardia; cardiotoxicity; cerebrovascular insufficiency; circulatory collapse (more common if injection too rapid); coarsening of the facial features; constipation; delirium; Dupuytren's contracture; encephalopathy; granulocytopenia; groin tingling; hair changes; hepatic disorders; hyperglycaemia; hypersensitivity; insomnia; leucopenia; lymphadenopathy; nephritis tubulointerstitial; Peyronie's disease; pneumonitis; polyarteritis nodosa; polyarthritis; purple glove syndrome; respiratory disorders; sensory peripheral polyneuropathy; suicidal behaviours; thrombocytopenia; tonic seizure; ventricular conduction depression (more common if injection too rapid); ventricular fibrillation (more common if injection too rapid) Side-effects, further information Fosphenytoin has been associated with severe cardiovascular reactions including asystole, ventricular fibrillation, and cardiac arrest. Hypotension, bradycardia, and heart block have also been reported. The following are recommended: monitor heart rate, blood pressure, and respiratory function for duration of infusion; observe patient for at least 30 minutes after infusion; if hypotension occurs, reduce infusion rate or discontinue; reduce dose or infusion rate in elderly, and in renal or hepatic impairment.
Interactions
**Severe interactions:**
- Inhibition of phenytoin metabolism may produce significant increases in plasma phenytoin concentrations and increase the risk of phenytoin toxicity.
**Other interactions (27):**
- Drug interactions which may occur following the administration of Pro-Epanutin are those that are expected to occur with drugs known to interact with phenytoin.
- Phenytoin metabolism is saturable and other drugs that utilise the same metabolic pathways may alter plasma phenytoin concentrations.
- There are many drugs which may increase or decrease plasma phenytoin concentrations.
- Equally phenytoin may affect the metabolism of a number of other drugs because of its potent enzyme-inducing potential.
- Determination of plasma phenytoin concentrations is especially helpful when possible drug interactions are suspected (see section 4.2).
- No drugs are known to interfere with the conversion of fosphenytoin to phenytoin.
Pregnancy
Important safety information For fosphenytoin sodium MHRA/CHM advice: Antiepileptics: risk of suicidal thoughts and behaviour (August 2008) See Epilepsy . MHRA/CHM advice: Antiepileptic drugs: updated advice on switching between different manufacturers' products (November 2017) See Epilepsy . MHRA/CHM advice: Antiepileptic drugs in pregnancy: updated advice following comprehensive safety review (January 2021) See Epilepsy .
Breast feeding
Small amounts present in milk, but not known to be harmful.
Hepatic impairment
Manufacturer advises caution-monitor free plasma-phenytoin concentration (rather than total plasma-phenytoin concentration) in hepatic impairment or hypoalbuminaemia and in hyperbilirubinaemia. Dose adjustments Manufacturer advises consider a 10-25% reduction in dose or infusion rate (except in the treatment of status epilepticus) in hepatic impairment or hypoalbuminaemia.
Renal impairment
Caution-monitor free plasma-phenytoin concentration (rather than total plasma-phenytoin concentration) in renal impairment or hypoalbuminaemia. M Dose adjustments Consider a 10-25% reduction in dose or infusion rate (except in the treatment of status epilepticus) in renal impairment or hypoalbuminaemia. M
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Clinical governance
**Author:** MedNext Clinical Team. **Clinical reviewer:** Dr Shameer Deen, MBBS, MS, MRCS. **Sources:** BNF, Indian Pharmacopoeia, CIMS India. **Correction:** Report errors at support@mednext.academy.
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