Antiepileptic (sodium channel blocker) Pregnancy: Category D — teratogenic (fetal hydantoin syndrome). Use only if essential.

Phenytoin

Brand names: Epanutin

Adult dose

Dose: Loading: 20 mg/kg IV (max 2000 mg); maintenance 300 mg/day

Route: IV (slow infusion) or oral

Frequency: Loading: single dose; Maintenance: once daily or divided

Max: Loading: 2000 mg; Maintenance: 600 mg/day

IV loading: max rate 50 mg/min (hypotension and arrhythmia risk). Oral maintenance: usual 300 mg/day in 1–2 divided doses. Non-linear kinetics — small dose changes cause large level changes. Now largely replaced by levetiracetam or fosphenytoin for acute seizures.

Paediatric dose

Dose: 20 mg/kg

Route: IV

Frequency: Loading: single dose; maintenance: 5 mg/kg/day divided BD

Max: Loading: 2000 mg; Maintenance varies by age

Concentration: 50 mg/ml

1 month–11 years: loading 20 mg/kg IV (max 2000 mg) at max 1 mg/kg/min. Maintenance: 2.5–5 mg/kg twice daily.

Dose adjustments

Renal

Monitor phenytoin levels; protein binding altered in renal failure.

Hepatic

Use with caution; reduced metabolism; monitor free phenytoin levels.

Paediatric weight-based calculator

Patient weight (kg)

1 month–11 years: loading 20 mg/kg IV (max 2000 mg) at max 1 mg/kg/min. Maintenance: 2.5–5 mg/kg twice daily.

Clinical pearls

Narrow therapeutic index: target level 10–20 mg/L (total)
Saturated kinetics: small dose increases can cause toxicity
IV formulation alkaline — flush line with saline only (precipitates with dextrose)
Fosphenytoin is water-soluble prodrug — preferred for IV use
Long-term use causes folic acid deficiency — supplement

Contraindications

Sinus bradycardia
Sino-atrial block
Second/third-degree heart block
Adams-Stokes syndrome
Porphyria

Side effects

Gingival hyperplasia (long-term)
Hirsutism
Coarsening of facial features
Nystagmus, ataxia, diplopia (toxicity)
Hypotension and arrhythmia during IV infusion
Stevens-Johnson syndrome (rare)
Folate deficiency

Interactions

Many significant interactions — enzyme inducer (CYP2C9, CYP3A4)
Warfarin — variable effect (monitor INR closely)
Oral contraceptives — reduced efficacy
Carbamazepine, valproate — complex interactions
Fluconazole, amiodarone — increase phenytoin levels

Monitoring

Phenytoin serum levels
ECG during IV loading
Blood pressure
LFTs
FBC
Folate

Reference: BNFc; BNF; NICE CG137 Epilepsies. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Eslicarbazepine acetate · Antiepileptic (sodium channel blocker)

Pathways