Reducing and enlarging formulas
Dr. Osama A. A. Ahmed
Reducing and enlarging formulas
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Factor (F) = Amount desired/ amount of formula
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From the following formula, calculate the quantity of each ingredient required to make
240 ml of calamine lotion:
Calamine
80 g
Zinc Oxide
80 g
Glycerin
20 ml
Bentonite magma
250 ml
Lime water (Calcium Hydroxide) to
1000 ml
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Factor (F) = 240/1000 = 0.24
Calamine
Zinc Oxide
Glycerin
Bentonite magma
Lime water to
Dr. Osama A. A. Ahmed
80 g
80 g
20 ml
250 ml
1000 ml
X 0.24
19.2 g
19.2 g
4.8 ml
60 ml
to make 240 ml
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Dilution, Concentration and Alligation
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Factor (F) = Amount desired/ amount of formula
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From the following formula, calculate the quantity of each ingredient required to make
1000 g of the ointment :
Coal tar
5 parts
Zinc Oxide
10 parts
Hydrophilic ointment
50 parts
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Total number of parts = 65
1000 g will contain 65 parts
Factor (F) = 1000/65 =
Coal tar
Zinc Oxide
Hydrophilic ointment
Dr. Osama A. A. Ahmed
5 parts
10 parts
50 parts
X 1000/65
76.92 g
153.85 g
769.23 g
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Selected calculation in contemporary compounding
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Constitution of dry powders for oral solution or suspension
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Some drugs as antibiotics lose their potency in a relatively short period when prepared
in a liquid dosage form.
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Example: the label of dry powder package of cefprozil (CEFZIL) for oral suspension
directs the pharmacist to add 72 ml of purified water to prepare 100 ml of suspension.
If the package contains 2.5 g of cefprozil, how many milligrams of the drug would be
contained in each teaspoonful dose of the constituted suspension?
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2.5 g = 2500 mg
2500 mg
X mg
Dr. Osama A. A. Ahmed
100 ml
5ml
X = 125 mg cefprozil
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Selected calculation in contemporary compounding
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Example: the label of a dry powder for oral suspension states that when 111 ml of water
are added to the powder, 150 ml of a suspension containing 250 mg of ampicillin per 5
ml are prepared. How many milliliters of purified water should be used to prepare, in
each 5 ml of product, the correct dose of ampicillin for a 60-lb. child based on the dose
of 8 mg/kg of body weight?
1 kg
2.2 lb
X kg
60 lb
X = 60/2.2 = 27.273 kg
ampicillin dose = 27.273 X 8 = 218 mg
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Total amount of ampicillin in the container
250 mg
5 ml
X mg
150 ml
X = 7500 mg
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The new volume to be prepared
218 mg
5 ml
7500 mg
X ml
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As the dry powder occupies 39 ml (150 ml – 111 ml ), so the amount of water added is
Amount of water = 172 – 39 ml = 133 ml
Dr. Osama A. A. Ahmed
X = 172 ml
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Selected calculation in contemporary compounding
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Constitution of dry powders for parenteral solution
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Example: Using a vial containing 200,000 units of penicillin G potassium, how many
milliliters of solvent should be added to the dry powder in preparing a solution having
a concentration of 25,000 units/ ml?
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25,000 units
200,000 units
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Use of prefabricated dosage forms in compounding
Example: Only capsules, each containing 25 mg of indomethacin, are available. How
many capsules should be used to obtain the amount of indomethacin needed in
preparing the following prescription?
R/ Indomethacin
2 mg/ml
Cherry syrup ad
150 ml
Sig. 5 ml b.i.d
Amount of indomethacin needed in the prescription = 150 X 2 = 300 mg
1 capsule
25 mg
X capsules
300 mg
X = 12 capsules needed
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Dr. Osama A. A. Ahmed
1 ml
X ml
X = 8 ml
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