Paste: Semisolid Topical Preparation
Pharmaceutical Pastes are semisolid dosage forms that contain one or more drug substances intended for topical/external application to the skin.
The term paste is applied to ointments in which large amounts of solids have been incorporated (e.g., zinc oxide paste) so they are generally very thick & stiff. They differ from ointments in their consistency, as they contain larger amounts of finely dispersed solids (20% to 50%) and so they are thicker and stiffer.
Paste form a protective coating over the area where they are applied & mainly used as antiseptic protective or smoothing dressing which is often spread on lint before being applied.
Pastes are also used in the treatment of oozing lesions, where they act to absorb serous secretions. Pastes also are used to limit the area of treatment by acting both as an absorbent and a physical dam.
Difference between Paste & Ointment
Base used for Pastes
Three types of bases are used for the preparation of pastes.
1. Hydrocarbon base
A mixture of Soft paraffin and liquid paraffin are used. ( e.g., zinc oxide paste, Aluminium paste compound)
2. Water miscible base
e.g. Resorcinol & sulfur paste
3. Water-soluble base/macrogels
Water-soluble bases are prepared from mixtures of high and low-molecular-weight polyethylene glycols (or macrogels).
PEG (mixture of high & low Mw Polyethylene glycols to get desired consistency) find more…
e.g. dental paste containing neomycin sulfate, Triamcinolone Dental paste B.P.C
Method of Preparation of Paste
Following methods are used to prepare pastes:-
b. Fusion methods
When the base is liquid or semisolid, the trituration method is used to prepare the paste.
When the base is semisolid and/or solid in nature, the fusion method is used to prepare the paste.
Preparation of Compound Zinc Paste (Using hydrocarbon base)
- Zinc oxide, finely sifted: 25 g
- Starch, finely sifted:25 g
- White soft paraffin 50 g
- (a) Zinc oxide and starch powder are passed through No. 180 sieve.
- (b) White 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 & uniform paste is obtained.