Types of solutions

Types of solutions

Syrup

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.

Elixir

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.

Tincture

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.

Oil-based solutions

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.

Miscellaneous solutions

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.

Dry or lyophilized mixtures for solution

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.