Displacement Values

• A suppository mould is filled by volume, but the suppository is formulated by weight.

• The volume of a suppository from a particular mould is uniform but its weight can vary when a drug is present due to difference in densities between the drug and base.

• The displacement value of a drug is the number of parts by weight of drug which displaces (occupies the same volume of) 1 part by weight of the base.

• Displacement values refer to values for theobroma oil. These values can also be used for other fatty bases.

• With glycerol-gelatin suppository base approximately 1.2g occupies the same volume as 1g of theobroma oil.

To calculate the displacement value of a drug:

Calculation of Displacement value:

If the displacement value of a particular drug is not known it can be calculated by the following method:

1. Prepare six suppositories using the base alone. Let the weight of these be A mg

2. Prepare six suppositories containing a known percentage of a drug. Let the weight of these be B mg

3. Calculate the amount of base present in the medicated suppositories. Let the weight be C mg

4. Calculate the amount of medicament present in the suppositories. Let the weight be D mg

5. Therefore, (A-C) will be the weight of the base displaced by the medicament.

6. Displacement value of the medicament for a particular base shall be:

Displacement value= D/(A-C)

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Example: 

Weight of six unmedicated suppositories = 6 g.

Weight of six suppositories containing 40% drug = 8.8 g.

Weight of base is then = 60% = 60/100 x 8.8 = 5.28 g.

Weight of drug in suppositories = 40% = 40/100 x 8.8 = 3.52.

Weight of base displaced by drug = 6 - 5.28 = 0.72 g.

If 0.72 g of base is displaced by 3.52 g of drug, then

1g of base will be displaced by 3.52/0.72g= 4.88 g.

Therefore displacement value of drug = 4.9 (rounded to one decimal place).


Q) Prepare six suppositories each containing 250 mg bismuth subgallate.


• Quantities are calculated for an excess of two suppositories. Therefore calculate for eight suppositories.


• DV of bismuth subgallate = 2.7


• A l g mould will be used with mould calibration = 0.94.

To calculate the amount of base required, a simple equation is used:
Amount of base = (N x y) – (N x D/ DV)

N=8 y = 0.94 D = 250 mg = 0.25 g DV = 2.7

Amount of base required

= (8 x 0.94) –((8x0.25)/2.7) = 7.52-0.741 = 6.779 9 = 6.78 g


Calculations for more than one drug:

Calculate the quantities required to make 15 suppositories each containing 150 mg hamamelis dry extract and 560 mg of zinc oxide. A 2g mould, with mould calibration of 2.04, will be used. Calculate for 17 suppositories (2 excess).

DV of hamamelis dry extract = 1 .5, DV of zinc oxide = 4.7.

Weight of hamamelis dry extract = 17 x 0.15 = 2.55 g.

Weight of zinc oxide = 17 x 0.56 = 9.52 g.

Weight of base = 17 x 2.04- (2.55/1.5 + 9.52/4.7) = 34.68- (1.7 + 2.03) = 30.95 g.

Calculation of quantities when the active ingredient is stated as a percentage

In this case, a displacement value is not required.

Q) Prepare eight suppositories containing 18% zinc oxide. Calculate for l g suppositories (2 excess). Mould calibration = 1

Weight of base required to fill mould = 10 x 1 = 10g.

Zinc oxide is 18% of total = 1.8g

Weight of base required= l0 -1.8 = 8.2 g.