Liquid Dosage Forms: Definition, Examples

Liquid dosage forms

Liquid dosage forms are the aqueous solutions of medicaments that are used to induce therapeutic effects, internally and externally. These forms are less stable than solids. Different routes of administration are used for liquid dosage forms, such as oral, intravenous, intramuscular, cutaneous, subcutaneous, etc. Liquid dosage forms are classified into two different categories namely, monophasic and biphasic.

Types of liquid dosage forms

Flowchart liquid dosage forms


Monophasic liquid dosage forms are liquid preparations containing only one base. These are also known as true solutions. A true solution is a clear homogenous mixture, prepared by dissolving a solid, liquid, or gas in a liquid phase. Monophasic liquid dosage forms are made for internal as well as external use. Syrups, mixtures, elixirs, and linctuses are meant for internal use, while gargles, mouthwashes, lotions, liniments, eye drops, ear drops, nasal drops, douches, and throat pains are meant for external use.

1. Syrups

Syrups are sweet viscous liquid formulations, intended for oral use. They are also termed as the concentrated or saturated solution of sucrose, as they contain 66.7 w/w amount of sugar in them. The presence of sugar in syrups makes them a valuable vehicle for the administration of nauseous substances. Purified water is preferred over potable water as potable water contains volatile and non-volatile impurities. Additives such as chemical stabilizers, coloring agents, flavoring agents, and preservatives are added to enhance the stability and shelf life of syrups. Syrups are mainly stored in colorless or amber-colored (light resistance container) bottles. Syrups are divided into two categories, medicinal and flavored syrups.


2. Mixtures

Mixtures are liquid dosage forms in which medicaments are dissolved in a suitable vehicle. These are manufactured for oral use and are usually supplied in three different-sized bottles (60ml, 120ml, and 240ml). Bottles containing a single dose are termed as draught. Ingredients of mixtures involve, water, aromatic water, and medicated vehicle, which acts as a vehicle for mixtures, and chemical stabilizers, coloring agents, flavoring agents, and preservatives are added as adjuncts (additives). Mixtures are not prepared to be stored for a longer period, as they are used to treat acute conditions like cough, diarrhea, constipation, and indigestion.

3. Linctuses

Linctuses are liquid oral doses, in form of syrups, developed to cure dry coughs and sore throats. Linctuses are viscous in comparison to syrups and contain medicaments such as demulcent, sedatives, and expectorants. Sucrose and glycerine are used as vehicles for linctuses. Additives, such as chemical stabilizers, coloring agents, flavoring agents, and preservatives are also used in the formation of linctuses. Codeine linctus, a syrup-like linctus, is widely used by pharmacists. They are supplied in air-tight glass bottles containing screw caps.


4. Elixirs

Elixers are clear hydroalcoholic liquids intended for oral use. Ethyl alcohol, water, glycerin or propylene glycol, flavoring agents, preservatives, and coloring agents are the main ingredients of elixirs. Elixirs are of two types, medicated and flavored. Medicated elixirs are used to induce therapeutic effects, while flavored ones are used as flavors and vehicles.


5. Gargles

Gargles are defined as liquid dosage forms of medicaments, intended for external use. These aqueous solutions are used to treat throat infections. These are diluted with warm water before their use, as per the directions given by the medical practitioner. The medication needs to be kept in contact with the mucous membrane of the throat for a few seconds before they are thrown out of the mouth. Phenol is present in small concentrations for its anti-bacterial activity, whereas potassium chlorate is added in some gargles for its weak astringent effects. Phenol gargles and potassium gargles are widely recommended by physicians for mild-throat infections.


6. Mouthwashes

Mouthwashes are aqueous solutions having a pleasant taste and odour to clean and deodorize the buccal cavity. They are often used for dental hygiene, but an antibacterial mouthwash can also be used to treat gum infections. The formulation of mouthwashes includes antibacterial agents, sweeteners, flavoring agents, alcohol, glycerine, and coloring agents.


7. Lotions

Lotions are defined as preparations meant for external use without friction. These are applied directly to the skin for cooling and protective purposes. They are also used for their anti-septic actions, for example, calamine lotions. Alcohol is also added in some formulations to enhance the cooling and soothing effect. eg., salicylic acid lotion. Colored fluted bottles are used for dispensing lotions.


8. Liniments

Liniments are liquid or semi-solid dosage forms, which are intended for external use. These are applied on the skin surface to provide analgesic, rubefacient, anti-irritant, and soothing effects. They are generally applied with rubbing or friction. Liniments should not be applied to the broken skin to avoid excess irritation. Liniments are available as alcoholic, oily solutions or as emulsions. They are dispensed in colored fluted bottles to differentiate them from preparations meant for internal use.

9. Eye drops

These are sterile dosage forms, which are intended for introduction in the eyes. Eye drops are used in small amounts and are administered on the surface of the eyes to treat mild infections. Eye drops contain saline in order to match the salinity of the eyes. Eye drops are commonly used to treat dry eyes or simple eye irritation, such as itching or redness.

Using Your Eye Drops | Seaphkl

10. Ear drops

Ear drops are liquid dosage medications, used for short-term treatments of the ear. These drops are meant to be instilled into the ear with the help of a dropper to induce therapeutic effects. These topical medications are useful in treating mild infections, reducing inflammation, and clearing ear wax.

Ear drops

11. Nasal drops

Nasal drops, as the name suggests, are aqueous solutions that are meant to be instilled in nose cavities with the help of a dropper. These drops are used to provide temporary relief from congestion caused in the nose by common cold, allergies, and hay fever.

12. Douches

A douche is a liquid dosage form of mediation intended to be instilled in body cavities. The word, ‘douche’ is generally used for vaginal solutions, but these douches are also used to irrigate other body cavities, such as eyes, ears, and nasal cavities. Their significant role is to remove the foreign particles present in the body. Apart from cleansing properties, douches also possess antiseptic and astringent properties. They are directed to be diluted with warm water before their use.

13. Throat-paints

Throat paints are viscous liquid formulations used to treat mouth and throat infections. Glycerin is used as a base and due to its viscous nature, the medication clings to the mucous membrane for a longer time. Glycerin also provides a sweet taste to the formulation. The most commonly used throat paints are boro glycerin, phenol glycerin, and tannic acid glycerin.

Throat paints


1. Suspensions

Suspensions are heterogeneous mixtures that are composed of finely divided solid particles, 0.5-5.0 in size, completely dispersed in a liquid or a semi-solid medium. The solid particles act as the dispersed phase, while the liquid phase act as the continuous phase. These mixtures can be easily separated through filtration. The particle size of the dispersed phase is kept smaller and fine, as it increases the rate of dissolution. These particles are apparent through the naked eye.

Suspensions are administered mainly through oral and parenteral routes. They are also used for external as well as ophthalmic use. Ophthalmic suspensions are rarely used as compared to eye drops. Additives added in the formulation of suspension are, flocculating agents, thickening agents(polysaccharides, inorganic agents), wetting agents, preservatives, and organoleptic additives. Suspending agents are also required in the formulation of suspensions to avoid aggregation of particles.


2. Emulsions

Emulsions are the heterogenous mixtures of two immiscible liquids, one of which is dispersed as minute globules into another. The globule size in emulsion varies from 0.25 to 25 μm. The minute globules phase is the dispersed phase, and the other in which minute globules are dispersed is termed as the continuous phase. Generally, two immiscible liquids cannot be dispersed for an extended period, therefore, emulsifying agents are added to the mixture for the formation of a stable emulsion. Emulsions cannot be separated through filtration. Emulsions are of two types, oil in water (o/w) and water in oil (w/o). Various excipients including emulsifying agents are added to form a stable emulsion.

a) Oil in water (o/w) type

Oil in water emulsions are generally meant for internal use, where oil act as the dispersed phase and water is the continuous phase. Emulsifying agents used in the preparation of such emulsions are, gum, acacia, tragacanth, methylcellulose, saponins, synthetic substances, and soaps formed + from monovalent bases (Na^{+}, K^{+}, NH_{4}^{+}) are used.

b) Water in oil (w/o) type

Water in oil type emulsions are generally meant for external use, such as lotions and creams. In these type of emulsions, water is the dispersed phase and oil is the continuous phase. Emulsifying agents used for such emulsions are, wool fat, resins, beeswax, and soaps from divalent bases (Ca^{++}, Mg^{++}, Zn^{++}) are used.


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