4.3.2 Compounding – Calculations
Using Commercial Tablets in Extemporaneous Compounding
In extemporaneous compounding, it may be necessary to use commercial tablet or capsule preparations to make some liquid and semi solid preparations. This is because they are convenient, quantitatively standardized, and available in a range of dosage strengths. Uncoated tablets are preferred, and controlled release dosage forms are generally unsuitable for compounding.
Tablets contain more than just the active ingredient.
Procedure for using tablets:
- Calculate the number of tablets required.
- If an even number of tablets are required, proceed to crush the tablets and combine with the suspending agent.
- If an uneven number of tablets are required, calculate the amount of powder needed.
- Weight the tablets and record the weight.
- Transfer the tables to the mortar and reduce to a powder.
- Weigh the correct amount of powder and use in compounding the preparation.
Examples
How many 500 mg paracetamol tablets are required to prepare 100 mL of the following elixir:
Paracetamol Elixir CF APF14
Paracetamol 120 mg
Ethanol (90% v/v) 0.7 mL
Propylene glycol 0.5 mL
Purified water 0.5 mL
Glycerol to 5 mL
Amount of paracetamol required:
120 mg in 5 mL
X in 100 mL
5 mL x X = 120 mg x 100 mL
X = (120 mg x 100 mL)/5 mL
X = 2400 mg paracetamol
Amount of tablets
2400 mg/500 mg = 4.8 tablets
Uneven number so need to calculate the amount of powder required. This also needs to take into account the amount of powder that sticks to the mortar. So in this case we will weigh 8 tablets.
In the laboratory take 8 tablets to the balance and weigh them = 4.447 grams.
Reduce to a powder in the mortar and pestle.
To calculate the amount of powder we require:
8 tablets = 4000 mg (4 g) of paracetamol = 4.447 grams of powder
2.4 g paracetamol = X powder
2.4 g x 4.447 g = 4 g x X
(2.4 g x 4.447 g)/4 g = X
2.668 g = X
Therefore, to make 100 mL of Paracetamol Elixir CF APF14 we require 2.668 g of crushed paracetamol 500 mg tablets.
📚 Read/Explore
- Practise question:
- Part A. How many 300 mg trimethoprim tablets are required to prepare 100 mL of the following prescription:
Trimethoprim 10 mg/mL
Syrup to 100 mL
-
- Park B. If four trimethoprim tablets weigh 2536 mg (active+inactive), how much of the ground powder do you require?
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