🧠 The theory: the drug pushes out some base
A mould is calibrated to hold a fixed weight of base alone. Adding drug takes up room and displaces base, so you need less base than the mould's nominal capacity. The displacement value (DV) tells you the exchange rate: it is the weight of drug that displaces one part by weight of base. So the base displaced is the total drug divided by the DV, and the base you actually weigh out is the full-mould base minus that displaced amount.
base displaced = total drug / displacement value
The four-step method
- •Step 1. Base with no drug = number of suppositories x the mould size.
- •Step 2. Total drug = number of suppositories x drug per suppository.
- •Step 3. Base displaced = total drug / displacement value.
- •Step 4. Base required = base with no drug minus base displaced.
Worked example
Prepare 10 suppositories, each 200 mg of drug, in a 1 g mould. The displacement value is 4. How much base is needed?
- Step 1 — Base if there were no drug: 10 suppositories x 1 g mould = 10 g.
- Step 2 — Total drug across all ten: 10 x 200 mg = 2 g.
- Step 3 — Base displaced by that drug: divide the drug by the DV, 2 / 4 = 0.5 g.
- Step 4 — Base actually needed: 10 minus 0.5 = 9.5 g.
Answer: 9.5 g of base
Worked example
Prepare 8 suppositories, each 250 mg of drug, in a 2 g mould. The displacement value is 2. How much base is needed?
- Step 1 — Base with no drug: 8 x 2 g = 16 g.
- Step 2 — Total drug: 8 x 250 mg = 2 g.
- Step 3 — Base displaced: 2 / 2 = 1 g.
- Step 4 — Base required: 16 minus 1 = 15 g.
Answer: 15 g of base
Harder: allowing for excess
In practice some mixture is always lost in the mould, so you prepare quantities for one or two more suppositories than you need. The method is identical, you just use the larger number throughout.
Worked example
To reliably fill 10, prepare quantities for 12 suppositories, each 100 mg of drug, in a 1 g mould. The displacement value is 1.5. How much base is needed?
- Step 1 — Base with no drug, using 12: 12 x 1 g = 12 g.
- Step 2 — Total drug: 12 x 100 mg = 1.2 g.
- Step 3 — Base displaced: 1.2 / 1.5 = 0.8 g.
- Step 4 — Base required: 12 minus 0.8 = 11.2 g.
Answer: 11.2 g of base
💡 Keep the steps in order
Most marks here are lost by forgetting step 3 (dividing the drug by the displacement value) or by subtracting the drug weight directly instead of the displaced base. Write all four steps out every time.
Test yourself
10 suppositories, 100 mg each, 1 g mould, displacement value 2. How much base?
Answer: Base = 10 g; drug = 1 g; displaced = 1 / 2 = 0.5 g; base required = 10 - 0.5 = 9.5 g
12 suppositories, 200 mg each, 2 g mould, displacement value 4. How much base?
Answer: Base = 24 g; drug = 2.4 g; displaced = 2.4 / 4 = 0.6 g; base required = 24 - 0.6 = 23.4 g
6 pessaries, 100 mg each, 4 g mould, displacement value 1.25. How much base?
Answer: Base = 24 g; drug = 0.6 g; displaced = 0.6 / 1.25 = 0.48 g; base required = 24 - 0.48 = 23.52 g