Semisolid Dosage Forms
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Direct answer
Halfway between a solid and a liquid, semisolid dosage forms are made to stay exactly where they are applied — ointments, creams, pastes and gels on the skin, suppositories in body cavities. Their pharmaceutical character comes almost entirely from the base: oleaginous petrolatum occludes and softens dry lesions, absorption bases accept water, emulsion bases wash off, water-soluble polyethylene glycol bases vanish without grease, and cocoa butter melts at body temperature to release a suppository's drug. The two calculations the syllabus demands are displacement value for suppositories and the qualitative reasoning that matches a base to a lesion.
What you must remember
- Ointment base classes: oleaginous (petrolatum, white ointment — emollient, occlusive, not water-washable), absorption (anhydrous lanolin, hydrophilic petrolatum — can incorporate water), emulsion or water-removable (hydrophilic ointment, o/w type), water-soluble (polyethylene glycol bases).
- Creams: oil-in-water vanishing creams (stearic acid based, ~60-80 per cent water, vanish on rubbing) and water-in-oil cold creams (cleansing, moisturising); the continuous phase names the class.
- Pastes contain a high powder fraction (20-50 per cent) — stiff, protective, absorbent; zinc paste (Lassar's) is the classic.
- Gels are structured liquids — carbomer (Carbopol), methylcellulose or HPMC hydrogels; clear, easily washed, release drug readily.
- Suppository bases: fatty bases — cocoa butter (theobroma oil, melting 30-35 °C), semi-synthetic hard fats (Witepsol type); water-soluble — glycero-gelatin base, polyethylene glycol bases.
- Displacement value = weight of drug that displaces one part of base; formula: DV = weight of drug in each suppository divided by (weight of base in plain mould minus weight of base with drug).
- Manufacture: fusion method for meltable bases, trituration (spatulation) on an ointment slab for incorporation of solids; levigation with a few drops of liquid to wet the powder first.
- Packaging in collapsible tubes, storage in a cool place, and labelling as an external preparation unless the product is officially intended otherwise.
Choosing the base for the lesion
Prescribing logic selects the base before the drug. A dry, scaly, thickened lesion — chronic eczema, psoriasis — needs occlusion to hydrate keratin, so an oleaginous or absorption base under a dressing. A weeping, exudative lesion must not be greased; a water-soluble or o/w base, often as a paste or gel, absorbs and dries. Acute inflamed skin gets a cooling, non-occlusive lotion or gel; the evaporation of the aqueous phase is part of the therapy. A patient who will treat a hairy area needs a washable o/w cream; a barrier application before radiotherapy or around a stoma wants a stiff paste.
Suppositories follow the same logic through melting and dissolving: cocoa butter melts just below body temperature for prompt release but is brittle when overheated and leaks at body temperature in tropical climates — Indian practice leans on hard fat bases with better temperature tolerance. Glycero-gelatin bases dissolve slowly and glycerin itself is hygroscopic and mildly laxative; polyethylene glycol bases dissolve and release drug without melting but can be irritating to sensitive mucosa. For each, release requires the base to yield the drug to moist mucosa, which is why drug solubility in the base relative to the secretion governs release rate — a fact examiners probe as "factors affecting drug release from suppositories".
The displacement value, worked once
Mould a plain cocoa butter suppository; it weighs, say, 2 g. Now repeat with 0.36 g of drug incorporated: the filled mould weighs 2.16 g, of which 1.80 g is base. The drug has therefore displaced 2.00 − 1.80 = 0.20 g of base, and the displacement value = 0.36/0.20 = 1.8. Now the practical use: each suppository must contain the prescribed dose of drug, and if you ignore the displacement value every moulded suppository will contain less base than calculated and the total weight will be wrong — the error is small for potent drugs dosed in milligrams and large for drugs dosed in hundreds of milligrams, which is exactly when the calculation is demanded.
The memory trap is inverting the formula. Fix it physically: displacement value answers "how many grams of drug push aside one gram of base" — a value above 1 means each gram of drug occupies less volume than a gram of base; a value below 1 the opposite.
The two confusions that decide marks
First, cold cream versus vanishing cream — students swap the phases. Cold cream is water-in-oil: continuous oil phase leaves a greasy film that replenishes fat, classically used to remove makeup and protect in winter. Vanishing cream is oil-in-water: a stearic acid soap matrix with a high water content that rubs in clear and leaves no visible film. Second, the cocoa butter polymorph: melt it too far above its melting point and cool it carelessly and it solidifies in a metastable form melting above body temperature — suppositories that never melt in the patient. Controlled, minimal melting is the manufacturing discipline the fact implies.
Frequently asked questions
Classify ointment bases with one example of each.
Oleaginous — petrolatum; absorption — anhydrous lanolin; emulsion (water-removable) — hydrophilic ointment; water-soluble — polyethylene glycol ointment.
Differentiate vanishing cream and cold cream.
Vanishing cream is oil-in-water, stearic acid based, high water content, rubs in without a visible greasy film; cold cream is water-in-oil, leaves a protective fatty film, used for cleansing and emollience.
Define displacement value and give its formula.
The number of parts by weight of drug that displace one part of the base; DV = weight of drug per suppository divided by (weight of base used alone minus weight of base used with the drug in the same mould).
Why should cocoa butter be melted at the lowest possible temperature?
Overheating converts it to a metastable polymorph that solidifies with a higher melting point, producing suppositories that fail to melt at body temperature and release drug poorly.
What are pastes and when are they preferred to ointments?
Stiff semisolids with 20-50 per cent powdered solid; preferred where absorption of exudate and a protective, durable film matter, as in zinc paste for weeping dermatitis.