The physician prescribed Phenytoin 0.2 g orally, twice daily. The medicine is available as 100 mg capsule. How many capsules will be administered for a dose?

Appeared in: SCTIMST Staff Nurse - 2015 (Set-A)

Explanation

The first step is to convert the prescribed dose from grams (g) to milligrams (mg) to match the unit of the available capsules. Since 1 g = 1000 mg, the prescribed dose of 0.2 g is equal to 200 mg.
The standard formula for calculating the number of tablets or capsules is (Desired Dose / Dose on Hand) × Quantity.
Plugging in the values: The Desired dose is 200 mg, and the dose on Hand is 100 mg per capsule.
Calculation: (200 mg / 100 mg) × 1 capsule = 2 capsules.

Option A: Administering 1 capsule would only provide 100 mg of Phenytoin, which is half of the prescribed 200 mg dose. This would result in underdosing the patient.
Option C: Administering 3 capsules would deliver 300 mg of Phenytoin. This is 100 mg more than the prescribed dose and would result in an overdose, increasing the risk of toxicity.
Option D: This is a common error. While 4 capsules is the total amount for the entire day (2 capsules per dose × twice daily), the question specifically asks for the number of capsules per single dose.

Clinical Relevance

Nursing practice connection: Knowing Drug Dose Calculation helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Accurate drug calculation is a fundamental and critical nursing skill. Errors can lead to significant patient harm, either from underdosing (ineffective treatment) or overdosing (toxicity).
This skill is a core component of the 'Six Rights of Medication Administration': Right Patient, Right Drug, Right Dose, Right Route, Right Time, and Right Documentation.
Phenytoin is an anticonvulsant with a narrow therapeutic index (10-20 mcg/mL). Even small deviations in dosage can lead to seizures (if underdosed) or toxicity (if overdosed), making precise calculation essential.

How to Approach the Question

First, carefully read the question to identify the prescribed dose (Desired), the available medication strength (Have), and the form (Quantity).
Note the units for both the prescribed and available doses. Here, they are different (grams and milligrams).
Convert the units so they are the same. It's usually easiest to convert the larger unit to the smaller one (g to mg).
Apply the standard dosage calculation formula: (Desired ÷ Have) × Quantity.
Finally, re-read the question to ensure your answer addresses the specific question asked (e.g., 'per dose' versus 'per day').

Concept Tested & Keywords

QCZp42ZnlU1SMHGJinhsxe

E6 Pharmacology Nurses Shanbhag 3e pp. 52-54, 51-53

E6 Pharmacology Katzung 16e p. 650-652

Practise the full SCTIMST Staff Nurse - 2015 (Set-A)

Attempt every question from this paper in a timed mock, then review the full solution for each one.