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Antiepileptic (sodium channel blocker)

Phenytoin

Brand names: Epanutin

Used in: Head Injury Seizures & Epilepsy

Phenytoin is an older antiepileptic still important for status epilepticus and some seizures; it needs careful handling because of its pharmacokinetics.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

For Status Epilepticus and Non-emergent Loading Dose: Adult loading dose is 10 to 15 mg/kg at a rate not exceeding 50 mg/min. ( 2.2 ) Pediatric loading dose is 15 to 20 mg/kg at a rate not exceeding 1 to 3 mg/kg/min or 50 mg/min, whichever is slower. ( 2.8 ) Continuous monitoring of the electrocardiogram, blood pressure, and respiratory function is essential. ( 2.2 ) Maintenance Dosing: Initial loading dose should be followed by maintenance doses of oral or intravenous Phenytoin Sodium Injection every 6 to 8 hours. ( 2.2 , 2.3 ) Intramuscular Administration: Because of erratic absorption and local toxicity, Phenytoin Sodium Injection should ordinarily not be given intramuscularly. ( 2.2 , …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2024-04-09. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Clinical monograph

How it works

It blocks voltage-gated sodium channels, stabilising hyperexcitable neuronal membranes.

Prescribing in practice

  • It has a narrow therapeutic index with saturable (zero-order) kinetics — small dose changes can cause large swings in level, so titrate against plasma levels (interpreted with albumin).
  • It is a potent enzyme inducer with many interactions, including reducing the effectiveness of hormonal contraception.
  • Intravenous use requires cardiac monitoring and a slow rate (risk of hypotension/arrhythmia) and causes severe tissue injury if it extravasates; long-term effects include gum overgrowth, hirsutism and neuropathy, and it is teratogenic.

Monitoring

Monitor plasma phenytoin levels (adjusted for albumin), ECG/blood pressure during IV use, and FBC/LFTs long-term.

Counselling the patient

  • Stay on the same brand/formulation, and attend for blood-level checks.
  • Report any rash promptly (stop and seek advice — serious skin reactions can occur), or gum swelling or unsteadiness.
  • It makes the contraceptive pill less effective.

Evidence & guidelines

Established for status epilepticus and epilepsy, used with plasma-level monitoring (NICE NG217).

Reference: NICE CG137 Epilepsies; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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