Cerebyx® dosing still a conundrum
From the Aug. 27,1997 Issue
It's been almost a year since product launch, and we are
still receiving reports about problems with Cerebyx®
(fosphenytoin). Many of the problems center around how doses
are supposed to be expressed. The "PE" designation is often
omitted from orders, leaving nurses and pharmacists to guess
what the prescriber meant. The following case illustrates
A pharmacist assumed that an order for a 5-year-old patient
for Cerebyx 100 mg was supposed to be 100 mg PE of Cerebyx,
and he sent the dose. Later that evening, a pediatric neurologist
gave a verbal order for Cerebyx 75 mg in AM and 150 mg in
PM. The pharmacist started to prepare Cerebyx 150 mg PE, but
another pharmacist stopped him. Since Cerebyx 150 mg is equivalent
to phenytoin 100 mg, and given the first order, it was possible
that the physician intended the patient to receive fosphenytoin
When contacted, the physician said that the order was for
Cerebyx 150 mg in the evening and 75 mg in the morning, equivalent
to 100 mg and 50 mg of phenytoin, respectively. He added that
there was no reason to call him because "that is the way you
In another situation, in response to an order for fosphenytoin
150 mg PE, a nurse gave three 2 mL vials of Cerebyx (300 mg
PE) instead of 1.5 vials (150 mg PE). She did not notice that
the label states "50 mg PE/mL" and in smaller print "2 mL."
The patient suffered no adverse effects.
The main problem with medication errors related to Cerebyx
is that FDA and Parke Davis are relying on the "perfectibility
model" (Leape LL. Error in medicine. JAMA 1994;272:1851-7)
which assumes that properly trained and motivated health professionals
won't make errors. But, as this case shows, despite the best
attempts that many of us have made with continuing education,
the errors won't stop because the basic systems used to communicate
proper dosing are flawed.
As probably thousands of other doctors have done, the physician
in the first case left off "PE" when he ordered it, which
led to an incorrect assumption by the pharmacist. In the second
case, the nurse fell victim to a familiar trap: a misleading
label. The label doesn't say "100 mg PE/2 mL." Rather, the
volume of the vial is listed too far apart from the strength
per milliliter ("50 mg PE/mL"), making it appear as though
only 50 mg were in the vial when "2 mL" isn't seen.
In our experience, when errors are tied to a product soon
after launch, they usually subside as people get to know the
product better. But the Cerebyx problem doesn't seem to be
subsiding. ISMP has contacted Parke-Davis about both of these
issues to offer assistance in resolving these problems. For
now, because errors with this product are fraught with danger,
no orders for fosphenytoin should be accepted if "PE" is omitted.
Further, to assure that proper check systems are in place,
the product should be removed from floor stock wherever possible.
If it must be kept in patient care areas, place it in bags
with this note "This 2 mL vial contains the equivalent of
100 mg of phenytoin (50 mg/mL)." Continue to promote dosing
in terms of "PE."