Pastes are semisolid preparations for external application containing a high proportion of finely powdered medicaments. They are stiffer and are usually employed for their protective action and for their ability to absorb serous discharges from skin lesions. They do not melt at ordinary temperature they form a coating over the affected area. Pastes are used as protective, antiseptic, and soothing dressings.
Differences Between Pastes and Ointments
(i) Pastes generally contain a large amount (50%) of finely
powdered solids. So, they are often stiffer than ointments.
(ii) When applied to the skin pastes adhere well, forming a
thick coating protects and soothes inflamed and raw surfaces and minimizes the
damage done by scratching in itchy conditions such as chronic eczema. It is
comparatively easy to confine pastes to the diseased areas. In contrast, ointments,
which are usually less viscous, tend to spread onto healthy skin, and this may
result in sensitivity reactions if the preparations contain a powerful
medicament such as dithranol.
(iii) Because the powder contents pastes are porous; hence,
perspiration can escape. Since the
powders absorb exudate, pastes with hydrocarbon base are less macerating than
ointments with a similar base.
(iv) They are less greasy than ointments but since their
efficacy depends on maintaining a thick surface layer they are far from
attractive cosmetically.
(v) Most pastes are unsuitable for treating scalp
conditions because they are difficult to remove from the hair.
Methods of Preparation:
Like ointment, pastes are prepared by trituration and fusion
methods. Trituration method is used when the base is liquid or semisolid.
The fusion method is used when the base is semisolid and/or
solid in nature.
Preparation 1:
Name: Compound Zinc Paste
Formula: Zinc oxide, finely sifted - 25 g
Starch, finely sifted - 25 g
White soft paraffin - 50 g
Type of preparation: Paste with a semi-solid base
prepared by fusion and trituration.
Procedure:
(a) Zinc oxide and starch powder are passed through the No.
180 sieve.
(b) Soft paraffin is melted in a water bath.
(c) The required amount of powder is taken in a warm mortar and
triturated with a little melted base until smooth. Gradually rest of the base
is added and mixed until cold.
Preparation 2:
Name: Zinc and Coal Tar Paste B.P.C.
Formula: Zinc oxide, finely sifted - 60 g
Coal tar - 60 g
Emulsifying wax – 50 g
Starch- 380 g
Yellow soft paraffin – 450 g
Type of preparation: Paste with a semi-solid base
prepared by fusion.
Procedure:
Method-I
(a) Emulsifying wax is melted in a tarred dish (70°C).
(b) The coal tar is weighed in the dish. Stirred to mix.
Soft paraffin is melted in a separate dish (70°C) and about
half is added to the tar-wax mixture; stirred well. The remainder is added; and
stirred again until homogeneous. Allowed
to cool at about (30°C) and zinc oxide (previously passed through 180 mesh) and
starch, in small amounts with constant stirring. Stir until cold.
Method-II
Wax and paraffin melted together, mixed well, and stirred
until just setting. Powders are mixed on a slightly warm tile and the tar is
incorporated. This method eliminates the risk of overheating.