fosphenytoin sodium at a glance
Medicine class
Antiseizure medicine; a prodrug of phenytoin
How it is given
By intravenous infusion, or in selected adults by intramuscular injection
Hospital monitoring
Heart rhythm, blood pressure and breathing are monitored during intravenous administration and afterwards
Prescription terminology
Amounts must be expressed as phenytoin sodium equivalents (PE) to reduce medication errors
What is fosphenytoin sodium used for?
The selected product is licensed in adults and children aged 5 years and over for:
Control of tonic-clonic status epilepticus.
Prevention and treatment of seizures associated with neurosurgery or head trauma.
Temporary replacement of oral phenytoin when oral administration is impossible or contraindicated.
How does fosphenytoin sodium work?
Fosphenytoin is rapidly converted into phenytoin. Phenytoin stabilises overactive nerve cells by modifying voltage-dependent sodium channels, limiting repetitive electrical firing and seizure spread.
How to take fosphenytoin sodium
Fosphenytoin sodium is prepared and administered by trained healthcare professionals. Intravenous administration requires controlled infusion and cardiorespiratory monitoring. Intramuscular administration is not used for status epilepticus and is not recommended in children. Patients do not administer it themselves.
Check that the prescribed amount is documented in phenytoin sodium equivalents (PE).
Report burning, tingling, pain, swelling or colour change around the injection site promptly.
If ongoing treatment is required, the clinical team may transfer you to an oral preparation when appropriate.
fosphenytoin sodium side effects
Common or expected effects
- Unusual eye movements or dizziness
- Itching, tingling or pins and needles
- Drowsiness, headache or unsteadiness
- Nausea or vomiting
- Injection-site pain or reaction
- Low blood pressure
Get urgent medical advice
- Severe allergic reaction, including breathing difficulty or facial or throat swelling
- Blistering, peeling or widespread rash; fever with swollen glands or organ symptoms
- Cardiovascular collapse, serious rhythm disturbance or respiratory depression
- Unexplained fever, pallor, bruising, bleeding or severe sore throat suggesting a blood disorder
- Spreading injection-site pain, swelling or discolouration, which may indicate purple glove syndrome
- Yellow skin or eyes, or other signs of acute liver injury
- Severe confusion, slurred speech or pronounced loss of coordination suggesting phenytoin toxicity
fosphenytoin sodium in pregnancy
Fosphenytoin is converted to phenytoin, which crosses the placenta and is associated with an increased risk of congenital malformations; effects on neurodevelopment cannot be excluded. Use requires specialist assessment, particularly before conception and during pregnancy. Do not stop treatment suddenly because breakthrough seizures may harm both the pregnant person and fetus. Pregnancy can also alter phenytoin concentrations, so additional clinical and blood-level monitoring may be needed.
fosphenytoin sodium while breastfeeding
Breastfeeding is not recommended while receiving the selected product. Although oral phenytoin reaches breast milk in low concentrations, it is unknown whether fosphenytoin itself is excreted in milk. Discuss feeding options with the specialist team.
fosphenytoin sodium interactions
Fosphenytoin has the extensive interaction profile of phenytoin. Medication review and, where appropriate, phenytoin-level monitoring are important when interacting treatments start or stop.
Delavirdine
Concomitant use is contraindicated because phenytoin may cause loss of antiviral response and resistance.
Azole antifungals, including fluconazole and voriconazole
They may increase phenytoin concentrations and toxicity risk.
Rifampicin or St John’s wort
They may lower phenytoin concentrations and reduce seizure control.
Common questions about fosphenytoin sodium
Answers are fully visible for fast scanning and source review.
Why is fosphenytoin sodium used instead of phenytoin injection?
Fosphenytoin is a water-soluble precursor that can be given intravenously or, in selected adults, intramuscularly. The body rapidly converts it into active phenytoin, but its preparation and administration differ from phenytoin injection.
Is fosphenytoin sodium used for status epilepticus?
Yes. The selected product is licensed to control tonic-clonic status epilepticus in adults and children aged 5 years and over, using intravenous administration.
Sources
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Prepared and reviewed by the iatroX editorial team.