Makindo Medical Notes

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.

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

• BNF: Fosphenytoin sodium