Fosphenytoin
💡 Doses are expressed as phenytoin equivalents (PE):
fosphenytoin sodium 1.5 mg = phenytoin sodium 1 mg.
🫀 IV use requires cardiac monitoring and immediate access to resuscitation equipment.
ℹ️ About
- 📚 Always verify dosing, infusion rates, contraindications, and monitoring in the
BNF (fosphenytoin).
- 🧑⚕️ Used mainly as a second-line agent for status epilepticus when benzodiazepines have been given.
- 🧪 Dosing is guided by clinical response and (where relevant) trough phenytoin levels.
Mode of action
- 💧 Water-soluble prodrug (ester) of phenytoin → can be infused IV more safely than phenytoin in many settings.
- 🔄 Rapidly metabolised by phosphatases to phenytoin (active drug).
- 🧠 Phenytoin blocks voltage-gated sodium channels (use-dependent) → stabilises neuronal membranes and reduces high-frequency firing.
- 🫀 Still carries risk of hypotension and arrhythmia → monitoring essential.
💉 Dose range (adult) — adjust to response and trough phenytoin concentration
| Name |
Starting Dose |
Frequency |
Route / rate |
| ⚡ Status epilepticus — loading |
20 mg(PE)/kg |
STAT |
IV infusion at 100–150 mg(PE)/minute (per local policy/BNF) |
| 🔁 Maintenance |
4–5 mg(PE)/kg/day |
Daily |
IV in 1–2 divided doses; infusion rate often 50–100 mg(PE)/minute |
| 👵 Elderly/frail: consider a 10–25% reduction in dose and/or infusion rate; titrate cautiously. |
| 🧾 Reminder: doses are PE (phenytoin equivalents). Fosphenytoin sodium 1.5 mg = phenytoin sodium 1 mg. |
Interactions
- 📌 See BNF (phenytoin is a strong enzyme inducer; interaction burden is significant).
- ⚠️ Expect interactions with warfarin, DOACs, oral contraceptives, many antimicrobials, and other antiepileptics.
Cautions
- 🫀 Heart failure, hypotension, conduction disease — monitor BP/ECG closely during infusion.
- 🫁 Risk of respiratory depression (especially with preceding benzodiazepines) → airway-ready environment.
- 💉 IV site issues: less tissue injury than IV phenytoin, but still avoid extravasation and use good cannula care.
- ✅ Cardiovascular complications are generally less frequent than IV phenytoin, but not zero.
Contraindications
- 🧬 Acute porphyria
- 🫀 2nd/3rd degree heart block, significant bradycardia / sinoatrial block (per BNF/label)
Side effects
- 🔥 Perineal burning/itching: can be intense but short-lived — warn patients before IV administration.
- 🥶 Chills; 😴 sedation; 😬 dysarthria; 🤕 headache; 😵 vertigo
- 📉 Hypotension; 🫨 movement disorders; 🍃 cerebellar signs (ataxia, nystagmus), especially with higher levels
- 🧪 Longer-term phenytoin effects (if continued): gingival hyperplasia, osteopenia, neuropathy, rash — review if converting to oral therapy.
References
