sample question

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Husseinmr's picture
Joined: 2009-01-31

TL, a 70-year old patient is currently taking phenytoin (Dilantin) 100 mg three
capsules daily at bedtime. TL can no longer swallow these capsules so the doctor
requests an equivalent dose of Dilantin suspension 125 mg/5 mL.
In order to maintain similar blood levels, you would give:
A. 5.5 mL in the morning and at bedtime
B. 6 mL in the morning and at bedtime
C. 7.5 mL in the morning and at bedtime
D. 11 mL daily at bedtime
E. 12 mL daily at bedtime
i seems like E but the answer they give is A

mitulbhalodia's picture
Joined: 2009-09-28

I aggree with you

siteadmin's picture
Joined: 2008-08-13
A tough calculation question

This is undoubtedly a tough calculation question because it requires you to know something specific that is not provided to you in the question. You need to know something about the drug and the formulations you are working with. Anytime you are working with a question that is switching a patient to an "equivalent" product, stop and ask yourself, are these two formulations actually equivalent or is some kind of adjustment required?

If you were able to look at a Dilantin monograph you would see that Phenytoin capsules and suspension are not equivalent in drug content or dosing. As we have been told we are working with Dilantin we know the capsules are made with Phenytoin sodium and the suspension is prepared with the free acid form of phenytoin. Our monograph tells us two things:

  1. "Because there is approximately an 8% increase in drug content with the free acid form over that of the sodium salt, dosage adjustments and serum level monitoring may be necessary when switching from a product
    formulated with the free acid to a product formulated with the sodium salt and vice versa"
  2. "Only extended phenytoin sodium capsules are recommended for once-a-day dosing."

Now that we know this information and will remember it for all future questions and practice ;) we can tackle the question.

We want a suspension that will deliver a total daily dose equivalent to 300 mg of Phenytoin sodium.

As our free acid form of phenytoin suspension will deliver 8% more of base Phenytoin we actaully want to give 8% less suspension, which gives us a total daily dose of 276 mg of Phenytoin free acid.

If we are going to use a Dilantin suspension of 125 mg/5 ml then we need 11ml of suspension daily to deliver 276 ml (11.04 rounded).

Generally Phenytoin is given in two or three equal doses. Looking at our options we can see twice daily is the only dosing option here. Therefore we are going to give a dose of 5.5 ml in the morning and at bedtime (option A).

Now that we know this important information we can use it for any future questions like this but it would be my hope that a question like this would give you access to the Phenytoin monograph. After all, in practice knowing this information is only half of the battle as we would also have to monitor serum drug levels and dose adjust the patient accordingly.




shoeb's picture
Joined: 2011-06-21

hey siteadmin can u tell me how 11 ml gives us 125/ml , i am unable to calculate and how many drugs we can prepare that have different dose in case of tablet and suspension..? please reply....