Semisolid dosage forms are products of semisolid consistency and applied to skin or mucous membranes for therapeutic or protective action or cosmetic function. They are intended for local drug delivery for treatment of conditions such as rashes, skin irritation, stings, fungal infections etc.
There are physical and physiological properties of semisolid dosage forms. Semisolid dosage forms are easily applicable with efficient drug release and high aqueous wash-ability. The main types of semisolid dosage forms are ointments, creams, pastes, poultices, and gels.
• Smooth texture • Elegant (neat) in appearance • Non-dehydrating • Non gritty • Non greasy and non-staining • Non hygroscopic
• Non-irritating • Do not alter membrane / skin functioning • Miscible with skin secretion • Have low sensitization effect
• Generally semisolid dosage forms are easily applicable with efficient drug release and high aqueous wash-ability
Ointments are semi-solid preparations for application to the skin or mucous membranes. Their basis is almost always anhydrous and generally contains one or more medicaments. They usually contain a medicament dissolved, suspended or emulsified in the base. The main rule of the local therapy is that if the skin is dry, use an ointment. Ointments are used for their emollient effect, protective action to the skin and for topical medication.
E.g.: • Antibiotics –bacitracin, neomycin, mupirocin, etc. • Antifungal agents –compound benzoic acid ointment • Anti-inflammatory agent- betamethasone valerate, hydrocortisone, etc.
They are viscous semisolid emulsion system intended for application to the skin i.e. for external use. Have opaque appearance compared with the translucent ointments. Consistency and rheological characters depend on whether the cream is w/o or o/w. Creams are of two types, aqueous creams and oily creams. In case of aqueous creams the emulsions are oil-in-water type (o/w) and in case of oily creams emulsions are of water-in-oil type (w/o).
O/W creams: Are composed of small droplets of oil dispersed in a continuous aqueous phase. Are more comfortable and cosmetically acceptable as they are less greasy and more easily washed off using water.
W/O creams: Composed of small droplets of water dispersed in a continuous oily phase. Are more difficult to handle but many drugs which are incorporated into creams are hydrophobic and will be released more readily from a w/o cream than an o/w cream. W/O are also more moisturizing as they provide an oily barrier which reduces water loss from the stratum corneum, the outermost layer of the skin.
Due to the presence of water soluble bases they can be easily removed from the skin. The aqueous creams have a tendency to grow bacterial and mold growth; therefore a preservative must be added in their formulation. E.g. cetomacrogol cream, cetrimide cream, hydrocortisone cream, zinc cream BPC. Properly designed W/O creams are elegant drug delivery system, pleasing in both appearance and feel after application. O/W creams are non-greasy and are rinsable (easily removed by washing).
Are semisolid preparations intended for external application containing a high proportion of finely powdered medicaments (about 50%). Pastes are basically ointments into which a high percentage of finely powdered insoluble solid (such as starch, zinc oxide, calcium carbonate etc.) has been added. The extraordinary amount of particulate matter stiffens the system.
Pastes are less penetrating than ointment. Paste make particularly good protective barrier when placed on the skin for, in addition to forming an unbroken film, the solid they contain can absorb and thereby neutralize certain noxious chemicals before they ever reach the skin. Like ointments, paste forms an unbroken relatively water impermeable film.
Unlike ointments, the film is opaque and therefore, an effective sun block accordingly. Because of their stiffness and impenetrability, pastes are not suited for application to hairy parts of the body. They are often spread on a lint or gauze rather than being applied directly to the skin.
Gels are transparent or translucent non-greasy semisolid aqueous preparations meant for external application to the skin or mucous membrane. Are prepared with the aid of a suitable gelling agents. Are often used when a non-greasy preparation is required for application to the scalp or skin. They are use d for medication or lubrication purposes.
They are used for lubricating catheters, surgical gloves and rectal thermometers. The gelling agents may be gelatin, or a carbohydrate such as gelatinized starch, tragacanth, sodium alginate or cellulose derivative. E.g. contraceptive jellies (spermicidal action), ichthammol jelly, fastum (ketoprofen) gel, Vivian (diclofenac) gel etc.
It is thick, soft, viscous, paste-like preparation for external use. Poultice must retain heat for a considerable time because they are intended to supply warmth to inflamed parts of body. Poultices act by increasing blood flow, relaxing tense muscles, soothing inflamed tissues, or drawing toxins from an infected area. Thus, they can be used to relieve the pain and inflammation associated with abscess; (e.g. Kaolin poultice B.P.C.)
How to use poultices
• The poultice is heated, with occasional stirring, until it can only just be tolerated on the back of the hand. • Then it is spread thickly on lint or other dressing and applied to the affected area which is sometimes first covered with muslin (very thin cloth) to facilitate removal after use. • A thick layer of cotton wool is applied to retain the heat and covering of oiled silk may be added to protect clothing.