Pastes are semisolid preparations for an 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 the 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 that 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 whereas 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 a 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 of the 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. The trituration method s used when the base is liquid or semisolid.
The fusion method is used when the base is semisolid and/or solid.
Name: Compound Zinc Paste
Formula: Zinc oxide, finely sifted – 60g
Starch, finely sifted – 25g
White soft paraffin -50g
Type of preparation: Paste with semi-solid base prepared by fusion and trituration.
(a) Zinc oxide and starch powder are passed through the No. 180 sieves.
(b) Soft paraffin is melted on a water bath.
(c) The required amount of powder is taken in a warm mortar, triturated with a little melted base until smooth. Gradually rest of the base is added and mixed until cold.
Name: Zinc and Coal tar Paste B.P.C.
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 semi-solid base prepared by fusion.
(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; stirred again until homogeneous. Allowed to cool at about (30°C) and zinc oxide (previously passed through 180 mesh) and starch, in a small amount with constant stirring. Stirred until cold.
Wax and paraffin melted together, mixed well, and stirred until just set. Powders are mixed on a slightly warm tile and the tar is incorporated. This method eliminates the risk of overheating.
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