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Understanding Phenytoin Equivalents


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, FAACT, FASHP

Bryan D. Hayes, PharmD, FAACT, FASHP

Chief Science Officer, ALiEM
Creator and Lead Editor, Capsules series, ALiEMU
Attending Pharmacist, EM and Toxicology, MGH
Assistant Professor of EM, Harvard Medical School
Bryan D. Hayes, PharmD, FAACT, FASHP

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  • Steve R


    What role do you see on a daily basis for the use of phenytoin. I often recommend a fosphenytoin load in our hemodynamically unstable patients as well as anyone with a “questionable” line as our acquisition costs are not significantly higher for fosphenytoin. Once the patient (if NPO) is off pressors and no longer receiving fluid boluses, I often try to switch back to phenytoin as I find the smaller maintenance doses of 100-150mg don’t quite have the same effect as the huge loading doses. However, always meet a lot of resistance from the “if it ain’t broke, don’t fix it” neurology clan. (Despite them being essentially the same drug…) wondering if you have any opinions on the utility of the fosphenytoin load with phenytoin maintenance?

    I also tend to continue with the IV therapy in patients receiving tube feeds, at least while critically ill. I never know how well it is really being absorbed, and the increase in nursing time / failure to meet caloric goals with holding feeds seems to be not worth it in the patient with a temporary feeding tube.

    Just some thought…

    • Bryan D. Hayes


      Our costs for fosphenytoin are also similar to phenytoin. We frequently use fosphenytoin for the load before converting to phenytoin maintenance doses. All of your points make sense and seem to reflect practice at my institution.