Solutions

33


Solutions




Study Points



image Definitions of solutions and expressions of solubility


image Advantages and disadvantages of using solutions


image Methods of controlling solubility


image Selection of vehicles


image Use of preservatives and other ingredients in solutions


image Principles of dispensing:


image Solutions for oral use


image Diluents


image Mouthwashes


image Nasal, oral and aural solutions


image Enemas


image Use of oral syringes




Solutions for oral dosage


Oral solutions are usually formulated so that the patient receives the usual dose of the medication in a conveniently administered volume, 5 mL or a multiple thereof, given to the patient using a 5 mL medicine spoon.


Advantages of solutions for oral use over a solid dosage form are that liquids are much easier to swallow than tablets or capsules and the medicament is readily absorbed from the gastrointestinal tract. Ease of taking is especially useful for children, elderly patients or those with chronic conditions such as Parkinson’s disease, who may have difficulty swallowing a solid oral dosage form. An advantage of solutions over suspensions is that the medicament is dispersed homogeneously throughout the preparation, without the need to shake the bottle. This makes the preparation easier for the patient to use and should ensure consistent dosage. Sometimes substances with a low aqueous solubility may be made into solution by the addition of another solvent rather than formulate the medicine as a suspension.


Disadvantages of solutions are that they are bulky, not convenient to carry around and less microbiologically and chemically stable than their solid counterparts. Oral solutions may have an unpleasant taste. The accuracy of oral dosage is dependent on the patient measuring the dose carefully.


The different forms of oral solutions are:



image Syrups, which are aqueous solutions that contain sugar. An example is Epilim® syrup (sodium valproate)


image Elixirs, which are clear, flavoured liquids containing a high proportion of sucrose or a suitable polyhydric alcohol and sometimes ethanol. Examples are phenobarbital elixir and chloral elixir (see Example 33.5)


image Linctuses, which are viscous liquids used in the treatment of cough. They usually contain a high proportion of sucrose, other sugars or a suitable polyhydric alcohol or alcohols. Examples are Simple Linctus BP and diamorphine linctus (see Example 33.4)


image Mixtures is a term often used to describe pharmaceutical oral solutions and suspensions. Examples are chloral hydrate mixture and ammonium and ipecacuanha mixture BP (see Example 33.3)


image Oral drops are oral solutions or suspensions which are administered in small volumes, using a suitable measuring device. A proprietary example is Abidec® vitamin drops.





Solutions for other pharmaceutical uses


Topical solutions for external use are considered in Chapter 36. Some topical solutions are designed for use in body cavities, such as the nose, mouth and ear.



Mouthwashes and gargles


Gargles are used to relieve or treat sore throats and mouthwashes are used on the mucous membranes of the oral cavity, rather than the throat, to refresh and mechanically clean the mouth. Both are concentrated solutions, although gargles tend to contain higher concentrations of active ingredients than mouthwashes. Both are usually diluted with warm water before use. They may contain antiseptics, analgesics or weak astringents. The liquid is usually not intended for swallowing. Examples are Phenol Gargle BPC and Compound Sodium Chloride Mouthwash BP (see Example 33.7). Proprietary examples are chlorhexidine (Corsodyl®) mouthwash and povidone-iodine (Betadine®) mouthwash.





Nasal solutions


Most nasal preparations are solutions, administered as nose drops or sprays. They are usually formulated to be isotonic to nasal secretions (equivalent to 0.9% normal saline) and buffered to the normal pH range of nasal fluids (pH5.5–6.5) to prevent damage to ciliary transport in the nose. The most frequent use of nose drops is as a decongestant for the common cold or to administer local steroids for the treatment of allergic rhinitis. Examples are normal saline nose drops and ephedrine nose drops, 0.5% or 1%. Overuse of topical decongestants can lead to oedema of the nasal mucosa and they should only be used for short periods of time (about 4 days) to avoid rebound congestion, called rhinitis medicamentosa. The nasal route may also be useful for new biologically active peptides and polypeptides which need to avoid the first pass metabolism and destruction by the gastrointestinal fluids. The nasal mucosa rapidly absorbs applied medicaments to give a systemic effect. There are some products utilizing nasal delivery currently available on the market, e.g. desmopressin (e.g. Desmospray®, DDAVP®), used in the treatment of pituitary diabetes insipidus. Accurate dosage is achieved using metered spray devices.



Ear drops


Ear drops are solutions of one or more active ingredient which exert a local effect in the ear, e.g. by softening earwax or treating infection or inflammation. They may also be referred to as otic or aural preparations. Propylene glycol, oils, glycerol (to increase viscosity) and water may be used as vehicles. Examples are aluminium acetate ear drops, almond oil ear drops and Sodium Bicarbonate Ear Drops BP (see Example 33.8).




Special labels and advice for nasal and aural preparations


Patients should be advised not to share nasal sprays or nose and ear drops in order to minimize contamination and infection. Manufactured nasal sprays and nose and ear drops will usually contain instructions for administration. Patients should be given advice on how to administer extemporaneously prepared nose and ear drops, accompanied by written information if possible (Fig. 33.1). For nose drops, it may be easier if the patient is lying flat with the head tilted back as far as comfortable, preferably over the edge of a bed. The patient should remain in this position for a few minutes after the drops have been administered to allow the medication to spread in the nose.



For ear drops, it may be easier for someone other than the patient to administer the drops. If desired, the drops can be warmed by holding the bottle in the hands before putting them in, but they must not be overheated. The ear lobe should be held up and back in adults, down and back in children, to allow the medication to run in deeper. They may cause some transient stinging. If the drops are intended to soften earwax, then the ears should be syringed after several days of use.


Extemporaneous preparations should be labelled with the appropriate expiry date following the official monographs. ‘For external use’ is not an appropriate label and so ‘Not to be taken’ is advised.



Enemas


Enemas are oily or aqueous solutions that are administered rectally. They are usually anti-inflammatory, purgative, sedative or given to allow X-ray examination of the lower bowel. Examples are arachis oil enema and magnesium sulphate enema. Retention enemas are administered to give either a local action of the drug, e.g. prednisolone, or for systemic absorption, e.g. diazepam. They are used after defecation. The patient lies on one side during administration and remains there for 30 min to allow distribution of the medicament. Microenemas are single-dose, small-volume solutions. Examples are solutions of sodium phosphate, sodium citrate or docusate sodium. They are packaged in plastic containers with a nozzle for insertion into the rectum. Large-volume (0.5–1 L) enemas should be warmed to body temperature before administration.


Enemas are packed in amber, glass bottles. Manufactured enemas will usually be packed in disposable polythene or polyvinyl chloride bags sealed to a rectal nozzle.





Formulation of solutions


Solutions comprise the medicinal agent in a solvent as well as any additional agents. These additional agents are usually included to provide colour, flavour, sweetness or stability to the formulation. Most solutions are now manufactured on a large scale although it may be occasionally required to make up a solution extemporaneously. When compounding a solution, information on solubility and stability of each of the solutes must be taken into account.


Chemical and physical interactions that may take place between constituents must also be taken into account, as these will affect the preparation’s stability or potency. For example, esters of p-hydroxybenzoic acid, which can be used as preservatives in oral solutions, have a tendency to partition into certain flavouring oils. This could reduce the effective concentration of the preservative agent in the aqueous vehicle of the preparation to a level lower than that required for preservative action.



Solubility


The saturation solubility of a chemical in a solvent is the maximum concentration of a solution, which may be prepared at a given temperature. For convenience, this is usually simply called solubility. Solubilities for medicinal agents in a given solvent are given in the British Pharmacopoeia (BP) and Martindale and other reference sources. Solubilities are usually stated as the number of parts of solvent (by volume) that will dissolve one part (by weight or volume) of the substance. In other situations, words are used to describe the solubility (see Examples 33.1 and 33.2). Using this information, it is often possible to calculate whether a solution can be prepared. Most solutions for pharmaceutical use are not saturated with solute.



Example 33.1


Potassium chloride is soluble in 2.8–3 parts of water.


This means that 1 g of potassium chloride will dissolve in 2.8–3 mL of water at a temperature of 20°C (taken as normal room temperature).

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Jun 24, 2016 | Posted by in PHARMACY | Comments Off on Solutions

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