Updated July 2026 · 9 min read

Fosphenytoin & Phenytoin: The ICU Nurse's Guide

Part of the ICU Emergencies Hub — browse every related guide in one place.

Phenytoin is the old guard of anticonvulsants — effective, but demanding. It runs slow, drops blood pressure, quarrels with the ECG, and needs its levels watched. Fosphenytoin is its better-behaved prodrug, made specifically to fix phenytoin's worst infusion problems. Both are dosed in "PE units," and getting that unit wrong is a classic error.

What they do

Phenytoin blocks voltage-gated sodium channels, stabilizing neuronal membranes and stopping the spread of seizure activity. Fosphenytoin is a water-soluble prodrug that the body rapidly converts to phenytoin; it exists mainly to solve phenytoin's IV problems — phenytoin's formulation is highly alkaline and irritating, which is what causes the tissue injury and much of the infusion-rate trouble.

PE units — the safety-critical detail

Fosphenytoin is dosed and ordered in phenytoin equivalents (PE): 1 mg PE of fosphenytoin delivers the equivalent of 1 mg of phenytoin. This convention exists so clinicians don't have to do a conversion at the bedside — but it also means every order, label, and pump entry must use PE. A loading dose is typically about 20 mg PE/kg. Confusing fosphenytoin milligrams with PE, or with phenytoin milligrams, is a well-known medication error.

Infusion rate and cardiovascular risk

The dangerous part of these drugs is the infusion. Given too fast, phenytoin causes hypotension and cardiac arrhythmias, including bradycardia and heart block — partly from the drug and partly from the propylene glycol diluent in IV phenytoin. Because of that, phenytoin has a strict maximum infusion rate, and the patient stays on continuous cardiac and blood-pressure monitoring throughout. Fosphenytoin can be given faster and with less local and cardiovascular irritation, which is a major reason it's often preferred for loading.

Dosing at a glance

Purple glove syndrome

IV phenytoin is caustic. Extravasation — or even properly running IV phenytoin — can cause purple glove syndrome, a progressive discoloration, edema, and pain distal to the IV site that can lead to serious tissue and limb injury. Fosphenytoin's water solubility largely removes this risk, which is another argument for it in a peripheral IV. Assess the site, and stop the infusion for any distal discoloration or pain.

Levels: free vs. total, and albumin

Phenytoin is highly protein-bound, so the "total" level can mislead you. What matters clinically is the free (unbound) phenytoin. In hypoalbuminemia — common in ICU patients — the total level underestimates the active free drug, so a "normal" total can actually be toxic, or a "low" total may be therapeutic. Either check a free level directly or use an albumin-correction (Sheiner-Tozer) calculation. Renal impairment further shifts binding. Toxicity shows as nystagmus, ataxia, slurred speech, and confusion.

Why levetiracetam often wins — but not always

Levetiracetam has displaced phenytoin as the go-to in many units precisely because it skips the rate limits, cardiac monitoring, tissue risk, interactions, and level-chasing. But fosphenytoin/phenytoin remains a validated, effective second-line agent, is preferred in some situations, and is still widely used — so knowing how to run it safely is not optional.

Bottom line

Fosphenytoin and phenytoin are effective second-line anticonvulsants that demand respect at the bedside: dose fosphenytoin in PE units and verify them, honor the infusion-rate limits with continuous cardiac and blood-pressure monitoring, guard peripheral sites against purple glove syndrome, and interpret levels as free drug corrected for albumin. Get those four things right and these older drugs still earn their place; get them wrong and they're among the more dangerous infusions a nurse runs.

This article is general educational information for licensed clinicians and students, not medical advice or a substitute for your institution's protocols, pharmacy guidance, or a provider's orders. Always follow facility policy and verify doses independently.