Liquid Dosage Forms
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Direct answer
When the patient is an infant, an elderly person, or anyone who cannot swallow a tablet, liquid dosage forms are the only realistic option — and they are also the chemist's hardest stability problem. The family divides into monophasic forms (solutions, syrups, elixirs, linctuses, spirits, tinctures, drops, gargles, mouthwashes, lotions, liniments and enemas) and biphasic forms (suspensions and emulsions). The examination anchors are numerical: IP simple syrup is 66.7 per cent w/w sucrose, elixirs carry 5 to 40 per cent alcohol, the dry-gum primary emulsion for fixed oils uses a 4:2:1 ratio of oil, water and gum, and a calibrated dropper delivers about 20 drops per millilitre.
What you must remember
- Simple Syrup IP contains 66.7 per cent w/w of sucrose — concentrated enough to be self-preserving by osmotic effect; USP syrup is 85 per cent w/v, a comparison examiners enjoy.
- Elixirs are clear, sweetened hydroalcoholic solutions with 5-40 per cent v/v alcohol — enough alcohol to keep oils and weakly water-soluble drugs in solution.
- Linctuses are viscous, sweet hydroalcoholic cough preparations meant to be sipped undiluted so they coat the throat.
- Emulsions are oil-in-water or water-in-oil; the dry gum (continental) primary emulsion ratios are 4:2:1 for fixed oils, 3:2:1 for mineral oils, 2:2:1 for volatile oils (oil:water:acacia).
- Suspensions dispense insoluble or foul-tasting drugs; they carry the label "shake well before use", and flocculated systems settle reversibly while deflocculated ones cake.
- Liniments are alcoholic or oily liquids for external rubbing; they must not be applied to broken skin, and containers carry "For external use only".
- Lotions are external suspensions or emulsions meant to soothe or cool (calamine lotion); gargles and mouthwashes treat the oropharynx.
- The official dropper is calibrated to deliver approximately 20 drops of an aqueous liquid per mL — household spoons vary, calibrated ones do not (5 mL per teaspoonful).
- Preservatives (benzoic acid, parabens, chlorbutol), antioxidants and buffers maintain stability; microbial growth is the constant threat once sucrose falls below self-preserving concentration.
Making a primary emulsion, step by step
The dry gum method is the practical exam's favourite because the ratios are mathematical and the failure modes are visible. Take arachis oil, a fixed oil, so the ratio is 4:2:1. Measure 4 parts oil by volume and 2 parts water. In a dry mortar, mix the oil with 1 part by weight of acacia powder and triturate; then add the 2 parts water all at once, and triturate energetically until the mixture whitens and crackles — the primary emulsion, a thick, creamy o/w core with the gum at the interface. Dilute this core, never the reverse order, with additional water in small portions to the final volume. If water meets gum before oil does, or is added slowly to the core, the gum clumps and the emulsion cracks.
The logic transfers to every question about emulsifiers: acacia at the interface lowers interfacial tension and forms a mechanical film; the required hydrophilic-lipophilic balance (HLB) tells you which emulgent suits which phase — higher HLB agents (tweens, 8-18) stabilise oil-in-water, lower HLB agents (spans, 3-6) water-in-oil. Creaming, the upward or downward drift of droplets, is reversible on shaking; cracking, the coalescence into separate layers, is not.
The confusion between the sweet vehicles
Syrup versus elixir is the recurring viva pair, and the differentiating sentence must be exact: a syrup is a concentrated aqueous sucrose solution without alcohol, for water-soluble drugs; an elixir is a hydroalcoholic solution whose 5-40 per cent alcohol dissolves drugs and flavour oils that water cannot. The follow-up trap is the "low-alcohol elixir" — reduce the alcohol and the flavour oils drop out of solution, so solubilisers or higher alcohol become necessary, a genuine formulation compromise worth stating.
The second confusion is between the two external forms: liniments are rubbed (often containing irritants like methyl salicylate or camphor, hence never on broken skin), while lotions are dabbed or applied without rubbing for a cooling, protective effect. Third, the dropper figure: the question "how many drops per millilitre?" expects approximately 20 for water — and the safe practice point, that paediatric drops are dosed by calibrated dropper and never by household spoon, converts the number into a clinical habit.
Frequently asked questions
What is the strength of Simple Syrup IP and why does it keep without preservative?
66.7 per cent w/w sucrose; the high sugar concentration exerts osmotic pressure that dehydrates microorganisms, making the syrup self-preserving.
State the primary emulsion ratios of the dry gum method.
Oil, water and gum in 4:2:1 for fixed oils, 3:2:1 for mineral oils and 2:2:1 for volatile oils, forming a stable primary emulsion before dilution to final volume.
Differentiate syrup and elixir.
Syrup: aqueous, sucrose-rich, alcohol-free, suits water-soluble drugs; elixir: clear hydroalcoholic (5-40 per cent v/v), suits drugs and flavouring oils needing alcohol for solubility, pleasantly sweetened.
Why do suspension labels carry "shake well before use"?
The insoluble particles settle on standing; shaking redistributes them for a uniform dose — without it, early doses are under-strength and the last dose is a slurry.
What is the difference between creaming and cracking of an emulsion?
Creaming is reversible concentration of droplets into a layer that redisperses on shaking; cracking is irreversible coalescence into permanently separated phases.