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Most oral drugs have relatively small doses; thus, a diluent, like lactose,
commonly is added to provide the necessary bulk to completely fill the
prescribed capsules.
The steps used in calculating the proper capsule fill may be described as
follows:
STEP 1. Select an appropriate capsule size.
STEP 2. Fill the capsule shell separately with each drug and diluent, and
record the weights of each.
STEP 3. Calculate the diluent displacement weight for each drug, as
demonstrated in the following example problem.
STEP 4. Calculate the amount of diluent required per capsule.
STEP 5. Calculate the total quantities of each drug and the diluent needed
to fill all of the capsules prescribed.
NOTE: Some pharmacists calculate for an extra capsule or two so as not to
run short of fill due to any powder residue remaining in the mortar after the
mixing process; this may not be done for “accountable” drugs, such as
narcotics.
Example calculation to determine a capsule fill
Determine the total quantities of each drug and lactose required to fill the
following prescription:
STEP 1. For the purpose of this example, assume the pharmacist selected a
size 1 capsule.
STEP 2. The pharmacist filled a capsule individually with each
ingredient, weighed them, and found:
STEP 3. The diluent displacement weights for drugs A and B are calculated
by ratio and proportion as follows:
For drug A:
For drug B:
STEP 4. The diluent required per capsule:
STEP 5. The total quantities of each drug and diluent needed to fill all the
capsules prescribed:
Suppository molding
6
Pharmacists extemporaneously prepare suppositories by using a mold, such
as that shown in Figure 17.4. The drug(s) prescribed and a suppository base
are the components of any suppository.
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FIGURE 17.4 An opened aluminum suppository mold that
prepares twelve 2-g suppositories in a universal shape for rectal or
vaginal use. When assembled, the threaded posts and wingnuts
secure the mold in precise alignment to receive the liquid fill.
When the fill is solidified, the mold is opened for easy removal of
the formed suppositories. (Courtesy of Total Pharmacy Supply.)
As defined in Appendix B, suppositories are solid dosage forms intended
for insertion into body orifices where they soften, melt, or dissolve,
releasing their medications to the surrounding tissues. Any of a number of
suppository bases can be used as vehicles for the medication; in
extemporaneous compounding, cocoa butter (also termed theobroma oil) is
commonly used. Cocoa butter is a solid at room temperature but melts at
body temperatures.
The calculations involved in preparing suppositories by molding are
described by the following steps. Other methods are used and may be found
in the cited reference.
7
To calibrate the suppository mold:
STEP 1. Fill all the cavities of the suppository mold with melted base.
After allowing time to cool and harden, extract the formed
suppositories, weigh them, and determine the total and average
suppository weights.
STEP 2. Divide the total and average suppository weights by the density of
the suppository base to determine the volume capacity of the
suppository mold and the average volume of each cavity.
To calculate and prepare medicated suppositories:
STEP 1. Weigh the active ingredient for the preparation of a single
suppository.
STEP 2. Mix the single dose of active ingredient with a portion of melted
base insufficient to fill one cavity of the mold (based on information
obtained by previously calibrating the mold).
STEP 3. Pour the drug–base mixture into a cavity, and add additional
melted base to completely fill the cavity.
STEP 4. After the suppository cools and hardens, extract and weigh it.
STEP 5. The weight of the base is determined by subtracting the amount of
the drug from the weight of the molded suppository.
STEP 6. The individual weights of the drug and base required to prepare
the prescribed number of suppositories may then be determined by
multiplying the amounts for a single suppository. The volume of base
required may also be calculated, if desired, by the use of its known
density.
Example calculation to prepare suppositories by
molding
Calculate the quantities of drug A and cocoa butter needed to fill the
following prescription:
STEP 1. 350 mg of drug A is weighed.
STEP 2. Because a rectal suppository mold prepares suppositories
weighing approximately 2 g, the amount of cocoa butter to use that would
be insufficient to fill one cavity may be estimated by:
2 g – 0.35 g (drug A) = 1.65 g (cocoa butter)
1.65 g ÷ 0.86 g/mL (density of cocoa butter) = 1.92 mL (approximate
volume of melted cocoa butter, when added to drug A, to completely
fill the cavity)
By mixing the drug with 1 mL of melted cocoa butter, the pharmacist knows
that this volume is insufficient to fill a cavity.
STEP 3. The mixture of 350 mg of drug A and 1 mL of melted cocoa
butter is placed in a cavity, and sufficient additional melted cocoa butter is
used to completely fill the cavity.
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STEP 4. The cooled and hardened suppository is extracted and is found
to weigh 1.95 g.
STEP 5. The weight of the cocoa butter in one suppository is calculated:
STEP 6. The total quantities of drug A and cocoa butter needed to fill the
prescription are:
NOTE: Some pharmacists calculate for an extra suppository or two so as not
to run short of fill mixture. In filling the mold, each cavity should be
slightly overfilled to allow for contraction on cooling.
Compounding Specialized Formulas
Not all commercially available products are suitable for every patient. On
occasion, a pharmacist must modify an existing product or prepare an
original formulation to meet the requirements of a patient.
In their compounding practices, pharmacists may develop their own
formulas, or they may refer to contemporary formulas developed and
published by colleagues.8 In order to facilitate pharmacy compounding,
some specialty companies and professional associations make available
instructional programs, resource materials, compounding equipment, bulk
active therapeutic agents, and compounding vehicles for the preparation of a
range of dosage forms.
9
Example calculations of specialized formulas
1. Calculate the number of tablets containing the combination of
spironolactone 25 mg and hydrochlorothiazide 25 mg that must be
used to prepare the following formula using Ora-Plus as the oral
suspending vehicle.
10
Spironolactone/hydrochlorothiazide:
1.
2.
2. Calculate the amount of FATTIBASE, a suppository base, required to
prepare 200 suppositories from the following formula for one
progesterone vaginal suppository11:
25 mg (progesterone) + 20 mg (silica gel) = 45 mg
2 g – 0.045 g = 1.955 g
1.955 g × 200 (suppositories) = 391 g FATTIBASE
CASE IN POINT 17.4
A pharmacist receives a telephone call from a pediatrician who has an
8.8-lb 1-month-old patient with an acid reflux condition. The pediatrician
prescribes famotidine (PEPCID) suspension at a dose of 0.5 mg/kg/day to
be delivered in a volume of 1 milliliter per dose. The pediatrician also
requests that 30 mL of the suspension be prepared.
The pharmacist uses finely crushed 20-mg famotidine tablets as the
source of the drug and a 1:1 mixture of Ora Plus:Ora-Sweet SF as the
vehicle.
How many 20-mg famotidine tablets are required?
PRACTICE PROBLEMS
Calculations for the Constitution of Dry Powders for Oral
Administration
After constitution of a dry powder, each 5 mL of ampicillin for oral
suspension contains 250 mg of ampicillin in package sizes to prepare
100 mL, 150 mL, or 200 mL of suspension. Which package size
should be dispensed for a 20-kg child who is to take 50 mg/kg/day
total, q.i.d., in equally divided and spaced doses for 10 days?
From the information in Figure 17.2, calculate the following: (a) the
volume of the original contents of the package upon dissolution; (b)
the content of doxycycline monohydrate, in mg; (c) the volume of
water to add to the original container if a doxycycline monohydrate
concentration of 10 mg/mL is desired; and (d) the concentration of
doxycycline monohydrate, in mg/5 mL, if 50 mL of water were used
rather than the directed 47.6 mL?
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3.
4.
5.
6.
7.
8.
The label on a bottle of dry powder mix for constitution states that
when 128 mL of water is added, 150 mL of an oral suspension
containing 250 mg of ampicillin in each 5 mL results.
a. How many milliliters of water should be added to the dry
powder mix if a strength of 150 mg of ampicillin per 5 mL is
desired?
b. If the dose of ampicillin is 5 mg/kg of body weight, how many
milliliters of water should be added to the dry powder mix so
that a child weighing 66 lb would receive the proper dose in
each 5 mL of the suspension?
Amoxicillin/clavulanate potassium (AUGMENTIN) powder for oral
suspension is prepared prior to dispensing by adding 134 mL of
purified water to the contents of the container to prepare 150 mL of
suspension. If each teaspoonful of suspension contains 125 mg of
amoxicillin and 31.25 mg of clavulanate potassium, how much of
each of these agents is contained in the dry powder prior to
reconstitution?
If, in Problem 4, a pharmacist wished to use the dry product to
prepare an oral suspension containing 200 mg of amoxicillin and 50
mg of clavulanate potassium/5 mL of suspension, how many
milliliters of purified water should be used for reconstitution?
Clarithromycin for oral suspension is available in bottles containing
dry granules intended for constitution with water to prepare a
suspension. The package insert instructs a pharmacist to add 27 mL of
water to prepare 50 mL of suspension for a clarithromycin
concentration of 125 mg/5 mL. Calculate (a) the weight of the dry
clarithromycin in the product. If the pediatric dose is 7.5 mg/kg every
12 hours, calculate the dose, in milliliters, for (b) a child weighing
16.7 kg and (c) another child weighing 55 lb.
Calculations Applied to Compounding for Parenteral
Administration
A vial contains 5 g of a powdered drug for reconstitution prior to use
in an infusion. The label states that when 9.6 mL of diluent is added,
the solution that results contains a drug concentration of 500 mg/mL.
A medication order calls for a drug concentration of 200 mg/mL.
How many milliliters of diluent should be added to the vial?
A hospital pharmacist constitutes a vial containing 2 g of piperacillin
sodium to 10 mL with sterile water for injection. This solution is then
diluted by adding it to 100 mL of 5% dextrose injection for
administration by infusion. What is the concentration, in milligrams
per milliliter (mg/mL), of piperacillin sodium in the infusion
solution?
9.
10.
11.
12.
13.
A vial of cefazolin injection contains 1 g of drug. When 2.5 mL of
diluent is added, 3 mL of injection is prepared. If 1.6 mL of the
injection is diluted to 200 mL with sodium chloride injection, how
many milliliters of the dilution should be administered daily to a child
weighing 40 lb if the daily dose is 25 mg/kg of body weight?
6
Medication Order: Piperacillin, 1500 mg every 6 hours.
Product: 3-g vial, piperacillin.
Pharmacy operations: Reconstitute vial with 14 mL of sterile water
for injection to prepare 15 mL of injection. Add portion required to 50
mL of sodium chloride injection.
Administer: 20-minute IV infusion.
a. How many milliliters of solution from the reconstituted vial
should be used?
b. What should be the infusion rate in milliliters per hour?
c. With a drop factor of 20 drops per milliliter, calculate the
infusion rate in drops per minute.
A medication order calls for 400 mg of cefazolin sodium to be
administered IM to a patient every 12 hours. Vials containing 500 mg,
1 g, and 10 g of cefazolin sodium are available. According to the
manufacturer’s directions, dilutions may be made as follows:
Explain how the prescribed amount of cefazolin sodium could be
obtained.
Using the vial sizes in Problem 11 as the source of cefazolin sodium,
how many milliliters of the diluted 500-mg vial should be
administered to a 40-lb child who is to receive 8 mg of cefazolin
sodium per kilogram of body weight?
Using cefazolin sodium injection in a concentration of 125 mg/mL,
complete the following table representing a Pediatric Dosage Guide:
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14.
15.
16.
19.
A vial contains 1 g of ceftazidime. Express the concentrations of the
drug, in milligrams per milliliter, following constitution with sterile
water for injection to the following volumes: (a) 2.2 mL, (b) 4.5 mL,
and (c) 10 mL.
Acetazolamide sodium is available in 500-mg vials to be constituted
to 5 mL with sterile water for injection before use. The dose of the
drug for children is 5 mg/kg of body weight. How many milliliters of
the injection should be administered to a child weighing 25 lb?
An intravenous infusion for a child weighing 60 lb is to contain 20
mg of vancomycin hydrochloride per kilogram of body weight in 200
mL of sodium chloride injection. Using a 10-mL vial containing 500
mg of vancomycin hydrochloride (dry powder), explain how you
would obtain the amount needed in preparing the infusion.
Calculations for the Use of Prefabricated Dosage Forms in
Compounding
17.
Using tablets, each containing 0.3 g of potassium permanganate,
explain how you would obtain the amount of potassium permanganate
needed for the prescription.
18.
How many 0.75-mg tablets of estropipate may be used to prepare the
prescription?
d
How many 30-mg tablets of testosterone are needed to fill the
prescription?
22.
20.
How many tablets, each containing 600 μg of scopolamine
hydrobromide, should be used in preparing the prescription?
21.
How many tablets, each containing 20 mg of hydrocortisone, should
be used in preparing the prescription?
How many milliliters of an injection containing 40 mg of a drug per
milliliter would provide the amount of the drug needed to prepare 120
mL of a 0.2% suspension?
23.
How many scored 100-mg allopurinol tablets may be used in
preparing the prescription?
24.
How many 20-mg tablets of enalapril, each weighing 120 mg, and
how many grams of lactose would be needed to prepare the
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