Understanding Pharmaceutical Liquid Dosage Forms

Liquid dosage forms are pourable pharmaceutical formulations which contain a mixture of active drug components and nondrug components (excipients) dissolved or suspended in a suitable solvent or mixtures of solvents. They are pharmaceutical preparations designed to provide the maximum therapeutic response in a target population with difficulty swallowing solid dosage forms and/or to produce rapid therapeutic effects.

Liquid dosage forms can be supplied as ready-to-use liquids or powders for reconstitution. They are administered by oral and parenteral (injectable, inhalation, ophthalmic, otic, nasal, and topical) routes. Oral liquids are nonsterile, whereas liquids administered by the parenteral route are available as sterile and nonsterile formulations.

This article details the various classifications of liquid dosage forms, excipients used in the formulation of liquid dosage forms, packaging and labelling of liquid dosage forms, quality control and assurance of liquid dosage forms as well as the advantages and disadvantages of liquid dosage forms.

Classification of Liquid Dosage Forms

Liquid dosage forms are broadly classified into two groups:

1. Monophasic liquid dosage forms
2. Biphasic liquid dosage forms

1. Monophasic liquid dosage forms

This is the simplest form of presenting medication for rapid and high absorption of medicinal products. It is a one-phase system consisting of two components, solute (the component that gets dissolved) and the solvent (the medium in which solute dissolves).

The monophasic liquid dosage forms are further classified into

1. Liquids for oral use e.g., mixtures, linctuses, draughts, elixirs, syrups, and drops
2. Liquids for external use e.g., lotions, liniments and collodions
3. Liquids for special use e.g. gargles, mouthwashes, throat paints, eye drops, eye lotions, ear drops, nasal drops and sprays, douches, enemas, inhalations and aerosols.
4. Parenteral solutions (injections)

i. Monophasic liquid dosage forms for oral use

This class of monophasic liquid dosage form comprises one phase pourable pharmaceutical formulations intended for oral use. Examples include mixtures, linctuses, draughts, elixirs, syrups, and drops.

a. Mixtures

Pharmaceutical mixtures are liquid oral preparation consisting of one or more medicaments dissolved, suspended or diffused an aqueous vehicle. They are usually freshly or recently prepared and are used fairly quickly, usually within a month for short term therapy like cough, diarrhoea, constipation etc.

Mixtures are further classified into five different groups

1. Simple mixtures containing soluble substances
2. Mixtures containing diffusible solids; that is, solids which do not dissolve in water, but may be mixed by shaking. As a result, it is evenly distributed throughout the liquid for sufficient time.
3. Mixtures containing indiffusible solids; that is, solids which are not dissolved in water and do not remain uniformly distributed in the solvent for sufficiently long time.
4. Mixtures containing precipitate forming liquids and
5. Mixtures containing slightly soluble liquids

Note:  The term ‘mixture’ may also be applied to a suspension.

b. Linctuses

Linctuses are viscous oral liquids containing one or more medicament dissolved in a vehicle that usually contains high proportion of sucrose or other sugars. They are chiefly used as demulcent, expectorant or sedative principally in the treatment or relief of cough. As such, linctus is intended to be sipped slowly in small doses and allowed to trickle down the throat in an undiluted form. This gives maximum and prolonged effect of medicament in the throat.

c. Draughts

A draught is an older term used to describe liquid oral preparations which contain only one or two large doses. The volume of the formulation is usually larger than that generally utilised in traditional mixture formulations and each dose is supplied in separate bottles.

d. Elixirs

Elixirs are clear, flavoured, sweetened, hydroalcoholic liquid oral preparations that usually contain either potent or unpleasant-tasting drugs. They may be medicated or nonmedicated. Compared to syrups, elixirs are usually less sweet and less viscous because they contain a lesser amount of sugar. Because of their hydroalcoholic character, elixirs are better able than are syrups to maintain both water-soluble and alcohol-soluble components in solution.

e. Syrups

Syrups are concentrated aqueous solutions containing one or more sugar components, chiefly sucrose, or sugar substitute. The concentration of sugar in syrup is 66.7 % W/W. Syrups may be medicated or nonmedicated. The nonmedicated often referred to as simple syrups are used as vehicles for medicinal substances to be added later, either in the extemporaneous compounding of prescriptions or in the preparation.

f. Drops (Paediatric drops)

Drops are liquid preparations of potent drugs usually in solution that are intended to be administered in small volumes with the aid of a suitable measuring device (calibrated dropper) to pediatric patients.

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