Cyclobenzaprine

1 The usual adult dose of cyclobenzaprine (Flexeril) is 10 mg 3 times a day. What would be the dose for an 8-year-old girl?

If the only information available forthe child is her age, Young's Rule or Cowling's Rule can be used to calculate an estimated dose.1

Young's Rule: age/(age + 12) × adult dose = dose for child

8/(8 + 12) × 10 mg = 4 mg 3 times a day

Cowling's Rule: age at next birthday/24 × adult dose = dose for child

9/24 × 10 mg = 3.75 mg 3 times a day

Notice that, in both of these equations, the child's age is in years.

A. You receive more information about this child and learn that she weighs 47 lbs. How much Flexeril should she receive per dose?

When the child's weight is available, you can use dark's Rule1 to provide a more accurate dose than the dose calculated based on the child's age.

Clark's Rule: weight/150 × adult dose = dose for child

For this equation, the child's weight is in pounds; and 150 is assumed to be the average adult weight in pounds.

47 lbs/150 lbs × 10 mg = 3.13 mg 3 times a day

B. You measure the child and find that she is 3'1'' tall. How much Flexeril should she receive per dose?

Since you have the patient's height and weight, you can find her body surface area (BSA) using a nomogram.2 Based on this information, the patient's BSA would be 0.71 m^sup 2^.

The dose based on BSA can then be calculated using the following equation:1

BSA/1.73 m^sup 2^ × adult dose = dose for child

For this equation, the child's BSA is in square meters, and 1.73 m^sup 2^ is assumed to be the average adult BSA.

0.71 m^sup 2^/1.73 m^sup 2^ 10 mg = 4.104 mg 3 times a day

C. You decide to use a dose of 4 mg based on the dose calculated above and prepare a cyclobenzaprine suspension with a concentration of 4 mg/tsp. How long will 120 ml of this suspension last?

120 mL × 4 mg/5 mL × 1 dose/4 mg × 1 day/3 doses = 8 days

D. What would be the percent strength and ratio strength of the suspension?

4 mg/5 ml × 1 g/1000 mg × 100 = 0.08% w/v

5 mL/4 mg × 1000 mg/g = 1250 mL/g = 1:1250 w/v

2 You need to prepare 20 pediatric suppositories, each containing 80 mg of drug. You know that the calibrated volume of the suppository mold is 1.2 mL; therefore, you weigh 80 mg of the drug and mix it with 500 mg of the suppository base you will be using to prepare the prescription. You melt this mixture and pour it into a cavity in your suppository mold. You also melt a small amount of the suppository base alone and pour it into the cavity to completely fill it. After the suppository has cooled, you remove it from the mold and find that it weighs 1.35 g. How much drug and suppository base will you need to prepare this prescription? (allow for two extra suppositories)

1.35 g/supp - 0.08 g drug/supp = 1.27 g of base/supp

80 m g/supp × 22 su pp × 1 g/1000 mg = 1.76 g of drug needed

1.27 g/supp × 22 supp = 27.94 g of base needed

3 The pediatric dose for ciprofloxacin after exposure to anthrax is 15 mg/kg every 12 hours for 60 days, not to exceed a daily dose of 500 mg.3 What would be the daily dose and total dose of ciprofloxacin over the course of therapy for a patient weighing 35 lbs?

15 mg/kg/dose × 1 kg/2.2 lbs. × 35 lbs × 2 doses/day = 477.27 mg/day

477.27 mg/day × 60 days × 1 g/1000 mg = 28.64 g total

4 A mother mistakenly buys Tylenol infant's drops rather than the Tylenol children's elixir for her 18-month-old child. She calls the pharmacy to ask a bout the difference in strength because she usualIy buys the elixir and gives her child a dose of 3/4 teaspoonful. However, she did not notice that she had bought the drops and gave her child the "usual" dose of 3/4 teaspoonful. The concentration of acetaminophen in Tylenol children's elixir is 160 mg/5 mL, and the concentration of acetaminophen in Tylenol infant's drops is 100 mg/mL.3 How much additional acetaminophen did the child receive from the Tylenol infant's drops?

3/4 tsp × 5 mL/tsp × 160 mg/5 mL = 120 mg from the usual dose of elixir

3/4 tsp × 5 mL/tsp × 100 mg/mL = 375 mg from drops

375 mg - 120 mg = 255 mg additional acetaminophen received

References

1. Ansel HC, Stoklosa MJ. Pharmaceutical Calculations. 11th ed. Baltimore, MD: Lippincott Williams &Wilkins; 2001: 73,76.

2. Lentner C, ed. Geigy Scientific Tables. 8th ed. West Caldwell, NJ: CibaGeigy Corp.; 1981: 227.

3. Short RM, Snitker JA, eds. CliniSphere 2.0 [book on CD Rom]. St. Louis, MO: Facts and Comparisons; 2004.

Shelly J. Prince, PhD, RPh

Southwestern Oklahoma State University

College of Pharmacy

Weatherford, Oklahoma.

Address correspondence to: Shelly J. Prince, PhD, RPh, College of Pharmacy, Southwestern Oklahoma State University, 100 Campus Drive, Weatherford, OK 73096.

Copyright International Journal of Pharmaceutical Compounding Jan/Feb 2005
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