Phenytoin / Fosphenytoin

How it works

It blocks voltage-gated sodium channels to stabilise neuronal membranes.

Prescribing in practice

• It has a narrow therapeutic index and non-linear (saturable) kinetics — small dose changes cause large level changes, so titrate carefully with level monitoring (interpreted alongside albumin).
• Intravenous loading must be given at a controlled rate with cardiac monitoring (risk of hypotension, arrhythmia and tissue injury).
• It is a strong enzyme inducer with many interactions; long-term use causes gum overgrowth; it is teratogenic.

Monitoring

Monitor phenytoin levels (corrected for albumin), ECG and blood pressure during IV loading, and FBC/LFTs; review interactions.

Counselling the patient

• Take it consistently; do not change brand or stop without advice.
• Keep good oral hygiene to reduce gum overgrowth.
• Report rash, unusual bruising, or yellowing of the skin/eyes.

Evidence & guidelines

Used for tonic-clonic seizures and as a second-line agent in status epilepticus, with strict level monitoring and IV-administration safeguards.