fosphenytoin image 1 (1)Sometimes, in an effort to make things simpler, we actually make them more confusing. Such is the case with phenytoin equivalents. 

Fosphenytoin is a water-soluble prodrug of phenytoin. After IV administration, much of the fosphenytoin is metabolized to phenytoin within 15 minutes. Advantages over phenytoin include the option for IM administration and less cardiotoxicity allowing for faster infusion rates. Even the potential for hyperphosphatemia from the release of phosphate is generally inconsequential. 

So, where is the confusion? 

If you want the patient to receive 500 mg of phenytoin, then you simply order fosphenytoin 500 mg PE (where PE stands for phenytoin sodium equivalents).

But… if you turn the vial around and look at the side, you’ll notice it says that each 10 mL vial contains fosphenytoin sodium 750 mg. Wait, so do we need to order fosphenytoin 750 mg to make sure the patient receives phenytoin 500 mg?

fosphenytoin image 2 (1)

Phenytoin Equivalent: Keeping it simple

Most, if not all, institutions have the process set up so that fosphenytoin is ordered as phenytoin equivalents. So, keep it simple.

  1. Choose your favorite phenytoin dosing calculator
  2. Calculate a dose
  3. Order that amount of fosphenytoin in PE units

I would have preferred to just learn a new weight-based dose for fosphenytoin, completely separate from phenytoin. But, this is what we have. Don’t over-think it. That’s where the confusion sets in.

Bryan D. Hayes, PharmD, DABAT, FAACT, FASHP

Bryan D. Hayes, PharmD, DABAT, FAACT, FASHP

Leadership Team, ALiEM
Creator and Lead Editor, Capsules and EM Pharm Pearls Series
Attending Pharmacist, EM and Toxicology, MGH
Associate Professor of EM, Division of Medical Toxicology, Harvard Medical School
Bryan D. Hayes, PharmD, DABAT, FAACT, FASHP


EM Pharmacist & Toxicologist @MassGeneralEM | Asst Prof @HarvardMed/@EMRES_MGHBWH | @ALiEMteam leadership | Capsules creator, ALiEMU | President, ABAT | #FOAMed