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 paediatric patients.

ii. Monophasic liquid dosage forms for external use

This class includes liquid preparation such as lotions, liniments and collodions

a. Lotions

Lotions are solutions, but may also be suspensions or emulsions, intended for external application to the skin. They are rubbed on the skin without friction with the help of some absorbent material such as cotton, wool or gauze soaked in it. In some cases, lotions are applied to the scalp, where the vehicle for the medication is alcohol based, allowing for rapid drying of the hair and thus making the product more acceptable to the patient (e.g. Salicylic Acid Lotion 2% BPC). In these cases, problems of flammability are addressed by suitable labelling.

b. Liniments

Liniments are liquid preparation intended to be rubbed with friction and massaged onto unbroken skin to obtain analgesic, rubefacient or generally stimulating effects. They are usually solutions of oils, alcohols or soaps, but may be formulated as emulsions.

c. Collodions

Collodions are principally solutions of pyroxylin in a vehicle of ether and alcohol that are intended to be painted onto the skin using a brush or rod and left to dry. When dry, the collodion leaves a protective film covering the site. Collodions are highly volatile and highly flammable and care should be taken to label any preparation appropriately.

iii. Monophasic liquid dosage forms for special use

These include

a. Gargles

Gargles are aqueous solutions containing antiseptics, antibiotics, and/or anaesthetics that are intended for prevention or treatment of throat infections. They are generally formulated in a concentrated form.

b. Mouthwashes

These are aqueous solutions with pleasant taste and odour intended for local treatment of the membranous lining of the mouth and gums. They generally contain antibacterial agents, alcohol, glycerine, sweetening agents, flavouring agents and colouring agents.

c. Throat paints

Throat paints are viscous liquid preparations that are applied with the help of a brush to the mucosa of the mouth or throat. Glycerine is commonly used as a base in throat paints because it possesses a sweet taste and it adheres to mucous membrane for a long period.

d. Eye drops

These are sterile solution or suspensions of drugs that are instilled into the eye with a dropper. The eye drops are usually made in aqueous vehicle. It should be isotonic with lachrymal secretions, buffered and free from foreign particles to avoid irritation to the eye.

e. Eye lotions

Eye lotions are sterile aqueous solutions used for washing the eye. They are supplied in concentrated form and are required to be diluted with warm water immediately before use. Eye lotions are generally used to remove foreign substances from the eye.

f. Ear drops

These are medicated solutions of drugs that are instilled in to the ear with a dropper. These are generally used for cleaning the ear, softening the wax and for treating the mild infections.

g. Nasal drops and sprays

Nasal drops are solutions of drugs that are instilled into the nostrils with the help of a dropper. Nasal sprays are the same preparations as nasal drops but instilled into the nostrils in the form of a spray. Both formulations intended for administration to the nasal cavities to obtain a systemic or local effect.

h. Douches

Douches are liquid preparations used to cleanse deodorize, soothe or medicate wounds, body orifices or cavities.

i. Enemas

Enemas are liquid preparations often formulated as solutions (though they may be presented as an emulsion or suspension) and are intended for rectal administration. They are used for cleansing, therapeutic or diagnostic purposes.

j. Inhalations

They are preparations containing volatile substances. They are used to relieve congestion and inflammation of the respiratory tract. They are added to hot water and the vapours are inhaled.

k. Aerosols

Aerosols are liquid preparations dissolved in a solvent. The solvent is a gas. It is delivered in the form of a spray. It is mainly used for treating asthma and also for migraine.

iv. Parenteral Solutions (Injections)

Parenteral solutions are sterile drug solutions intended for administration by needle or pressure syringe. Drugs may be injected into most any vessel or tissue of the body, but the most common routes are intravenous (IV), intramuscular (IM), and subcutaneous (SC).

Parenteral solutions may be small-volume injections, packaged in ampules for single-dose administration, or vials for multiple-dose injections. Large-volume parenterals containing 100 mL to 1 litre of fluid, are intended for the slow intravenous administration (or infusion) of medications and/or nutrients in the institutional or home care setting.

2. Biphasic liquid dosage forms

Biphasic liquid dosage form is one which contains two phases. A good example of such liquid dosage form is suspensions and emulsions.

i. Suspensions

Suspensions are biphasic liquid dosage forms containing essentially insoluble finely divided solid particles or drug(s) suspended with the help of a suspending agent(s) in a liquid medium. The solid particles act as disperse phase whereas liquid acts as a continuous phase.

ii. Emulsions

An emulsion is a biphasic liquid preparation containing two immiscible liquids (usually oil and water) one of which is dispersed as minute globules into the other and rendered homogeneous by the addition of an emulsifying agent. The liquid which is converted into minute globules is called the disperse phase and the liquid in which the globules are dispersed is called the continuous phase.

Emulsions are of two types

1. Oil in water type (O/W): Emulsion in which oil is in the dispersed phase whereas water is in the continuous phase.
2. Water in oil type (W/O): Emulsion in which water is in the dispersed phase whereas oil is in continuous phase.

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