Displacement value & Specific problems in formulating suppositories – Pharmaceutics – I B. Pharma 1st Semester




problems in formulating suppositories

Learning objectives

At the end of this lecture, student will be
able to:

the importance of the displacement value

the displacement value

problems encountered during suppositories formulation and suggest remedies for
the same

Explain the importance of lubricants used for
suppository moulds

Displacement value

q  The volume of a suppository from a
particular mould is uniform but its weight will vary because the densities of
medicaments usually differ from the density of the base.

q  Hence it becomes  necessary to calculate displacement value of
the base to be used for the particular medicament

Definition: The number of parts of medicament
(drug) that displaces one part by weight of the base is known as the
displacement value of that drug.

Calculation of Displacement

Prepare and
weigh 10 suppositories containing base alone = [A] g

 Prepare and weigh 10 suppositories containing
drug & base= [B]g

 Total weight of the drug [C] = B-A g

 Weight of the base displaced by [C] g of drug
= [D]                                                          

                                Displacement Value [D.V] = [C] / [D]                                                                     

v  Amount of the base displaced = Weight of the drug / Displacement value

Calculation for practical

To prepare and
submit 6 Boric acid suppositories (Calculate for 8)

Weight of boric acid for 1 suppository        =             120mg  

Weight of boric acid for 8 suppositories     =             960mg
= 0.96g

Weight of cocoa butter for 1 suppository    =             1g

Weight of cocoa butter for 8 suppositories =             8g

Displacement value of boric acid is 1.5 i.e., 1.5 g of boric
acid displaces 1 g of cocoa butter Therefore 0.96g of boric acid displaces =
0.96/1.5 = 0.64g of base

Therefore the total weight of cocoa butter required = 8 g –
0.64 g = 7.36 g

Lubrication of mould

q  If the cavities are imperfect, i.e.
poorly polished or scratched, it may be difficult to remove the suppositories
without damaging their surfaces

q  So lubrication of the moulds is

q  In case of greasy or oily base water
soluble lubricants are required

q  For water soluble /miscible bases
oily lubricant may be used

q  For cocoa butter the following
lubricant solution formula may be used:
           Soft soap              

q  For glycero-gelatin base, liquid
paraffin or arachis oil may be used as lubricant.

Specific problems in formulating suppositories

  1. Water
    in suppositories

  2. Hygroscopicity

  3. Incompatibilities

  4. Viscosity

  5. Brittleness

  6. Volume

  7. Lubricants

  8. Rancidity
    & oxidation

Water in suppositories

q  Water
is used as a solvent to incorporate a water-soluble substance in the
suppository base. Incorporating  water
should be avoided for the following reasons – 

accelerates the oxidation of fats

the water evaporates the dissolved substances crystallize out

presence of water, reactions between various ingredients of suppositories may

water may be contaminated with bacteria or fungus


q  Glycerinated
gelatin suppositories lose moisture in dry climates and absorbs moisture in
high humidity

q  Polyethylene
glycol bases are hygroscopic in nature


q  Poyethylene
glycol (PEG) bases are incompatible with silver salts, tannic acid, quinine,
aspirin, benzocaine and sulfonamides.

q  Many
chemicals have a tendency to crystallize out of PEG     e.g. sodium barbital, salicylic acid and


q  Viscosity
of melted base is low in cocoa butter

q  High
in PEG and glycerinated gelatin

q  Low
viscosity base when melted the suspended particles may sediment very quickly
producing non-uniform distribution of drugs

Remedies for viscosity related problems

q  The
base should be melted at the minimum temperature required to maintain the
fluidity of the base

q  The
base is constantly stirred in such a way that the particles cannot settle and
no air is entrapped in the suppository.

q  A
base with a narrow melting range closer to rectal temperature is used.

q  Inclusion
of approximately 2% aluminium monostearate increase the viscosity of the fatty
base and also helps in homogeneous suspension of particles.

q  Cetyl,
stearyl, myristyl alcohol or stearic acid are added to improve the consistency
of suppositories.


q  Cocoa
butter base is not brittle but synthetic fat bases (with high degree of
hydrogenetation and high stearate containing bases) are brittle

q  Brittle
suppositories produce trouble during manufacture, handling, packaging and
during use

q  Causes:
Rapid chilling (shock cooling) of the melted bases in an extremely cold mould

Remedies for Brittleness

q  The
temperature difference between the melted base and mould should be as small as

q  Addition
of small amount of Tween80, castor oil, glycerin or propylene glycol imparts
plasticity and make it less brittle

Volume contraction

q  When
the bases are cooled in the mould volume of some bases may contract. Volume
contraction produces

release facilitating the ejecting from mold.

hole formation at the top.

imperfection can be solved by adding slight excess base over the suppositories
and after cooled the excess is scrapped off.


q  Cocoa
butter adheres to suppository molds because of very low volume of contraction

q  Aqueous
lubricant may be used to remove the suppositories easily from the moulds

q  They
are applied by wiping, brushing or spraying

q  The
mould surfaces may be coated with teflon to reduce the adhesion of base to
mould wall

Rancidity & oxidation

Auto oxidation

Due to auto oxidation of unsaturated fatty acids
present in the base

Aldehydes ketones

saturated and unsaturated aldehydes, ketones and
acids may formed


Rancidity have very strong unpleasant odour

phenomenon is called rancidification


q  To
prevent this suitable antioxidants like



                a– and b-tocopherols,

(present in cotton seed oil),

(present in sesame oil)

gallate, gallic acid,

and tannic acids,

acid (Vit C.),

hydroxyanisole (BHA)

hydroxyanisole (BHA).


q  The number of parts of medicament
(drug) that displaces one part by weight of the base is known as the
displacement value of that drug.                                                             

v  Amount of the base displaced = Weight of the drug / Displacement value


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