Types of solutions: Syrup, Elixir, Tincture, Oil-based solutions, Miscellaneous solutions, Dry or lyophilized mixtures for solution
Types of solutions
Syrup
is a saturated sugar solution. Thus, aqueous solutions containing sugar at or
close to its saturation concentration of 67% w/w are called syr-ups. In special
circumstances, it may be replaced in whole or in part by other sugars (e.g.,
glucose/dextrose) or nonsugars (e.g., sorbitol, glycerin, and propylene
glycol). Syrups containing flavoring agents but no drugs are called
nonmedicated syrups. Syrups provide a pleasant means of admin-istering a liquid
form of a disagreeable tasting drug. Syrups are appropri-ate for water-soluble
drugs. Cold and cough syrups are the most common examples of medicated syrups.
Most
syrups contain the following components in addition to the puri-fied water and
drug(s): (a) sugar, usually sucrose or other sugar substitutes are used to
provide sweetness and viscosity, (b) antimicrobial preserva-tives, (c)
flavorants, and (d) colorants. Syrups may also contain solubilizing agents,
thickeners, or stabilizers.
Sucrose is the sugar that is
most frequently employed in syrups. Sucrose
not only provides sweetness and viscosity to the solution but it also
renders the solution inherently antimicrobial. Although dilute sucrose
solutions can provide an efficient nutrient medium for the growth of
microorganisms, concentrated sugar solutions are hypertonic and resist
microbial growth because of the unavailability of the water required for the
growth of micro-organisms. Glycine, benzoic acid (0.1%–0.2%), sodium benzoate
(0.1%– 0.2%), and various combinations of methylparabens, propylparabens, and
butylparabens or alcohol are commonly used as antimicrobial preservatives.
Most
syrups are flavored with synthetic flavorants or with naturally occurring
materials, such as orange oil and vanillin, to render the syrup pleasant
tasting. To enhance the appeal of the syrup, a coloring agent that correlates
with the flavorant employed (i.e., green with mint, brown with chocolate) is
used.
Sweetened
hydroalcoholic (combinations of water and ethanol) solutions are termed
elixirs. Compared to syrup, elixirs are usually less sweet and less viscous,
because they contain a lower proportion of sugar and are conse-quently less
effective than syrups in masking the taste of drugs. In contrast to aqueous
syrups, elixirs are better able to maintain both water-soluble and
alcohol-soluble components in solution due to their hydroalcoholic proper-ties.
These solubility characteristics often make elixirs preferable to syrups.
All
elixirs contain flavoring and coloring agents to enhance their palatabil-ity
and appearance. Each elixir requires a specific proportion of alcohol and water
to maintain all of the components in solution. Elixirs containing over 10%–12%
alcohol are usually self-preserving and do not require the addition of antimicrobial
agents for preservation. Alcohols precipitate tragacanth, acacia, agar, and
inorganic salts from aqueous solutions. Therefore, such substances should
either be absent from the aqueous phase or present in such low con-centrations
so as not to promote precipitation on standing. Examples of some commonly used
elixirs include dexamethasone elixir USP, phenobarbital elixir, pentobarbital
elixir USP, diphenhydramine HCl elixir, and digoxin elixir.
Tinctures
are alcoholic or hydroalcoholic solutions of chemical or soluble constituents
of vegetable drugs. Most tinctures are prepared by the extraction process.
Depending on the preparation, tinctures contain alcohol in amounts ranging from
approximately 15% to 80%. The alcohol content protects against microbial growth
and keeps the alcohol-soluble extractives in solution. Because of the alcoholic
content, tinctures must be tightly stoppered and not exposed to excessive
temperatures to avoid or minimize the evaporation of alcohol.
Although
most solution dosage forms are aqueous based, certain solutions are oil based.
For example, progesterone injection is a solution of the hor-mone in a suitable
vegetable oil for intramuscular use. In addition, solution of cyclosporine A in
olive oil is available for ophthalmic and oral use. Oily solutions are
generally not preferred as oral dosage forms due to palatability concerns. When
a drug needs to be administered as a solution in oil, dosage forms such as
emulsions and self-emulsifying or self-microemulsifying drug delivery systems
are preferred. These systems contain high concentrations of surfactant and
sometimes also ethanol so that they spontaneously make an emulsion or a
microemulsion upon gentle mixing with water.
Hydroalcoholic
solutions of aromatic materials are termed spirits.
Mouth-washes are solutions used to
cleanse the mouth or treat diseases of the oral membrane. Antibacterial
topical solutions (e.g., benzalkonium chloride and strong iodine) kill
bacteria when applied to the skin or mucous membrane.
Some
drugs, particularly certain antibiotics, have insufficient stability in aqueous
solution to withstand long shelf lives. Thus, these drugs are for-mulated as
dry powders or granule dosage forms for reconstitution with purified water
immediately before dispensing to the patient. The dry pow-der mixture contains
all of the formulation components including drug, flavorant, colorant, buffers,
and others, except for the solvent. Once recon-stituted, the solution remains
stable for the in-use period. Examples of dry powder mixtures intended for
reconstitution to make oral solutions include cloxacillin sodium, nafcillin
sodium, oxacillin sodium, and penicillin V potassium. Several proteins and
antibody–drug conjugates are commer-cially available as lyophilized solids in
vials for reconstitution into solutions immediately before parenteral
administration.
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