This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
The chemicals and the various mineral, plant, or animal crude drugs may be employed in medicine as such without change, e. g., sodium bicarbonate or cod-liver oil, or powdered digitalis leaves; or they may be made into pharmaceutic preparations, as the rhubarb and soda mixture, the emulsion of cod-liver oil, or the tincture of digitalis.
Pharmaceutic preparations are the prepared forms into which drugs are made for convenient employment in medicine. It is not convenient, for instance, to administer cinchona in the form of cinchona bark. It would be a disagreeable task for a patient to chew the bitter bark, and difficult, because of the inert matter present, to obtain in this way the full physiologic activity of the drug. But the tincture of cinchona, a pharmaceutic preparation, represents the full physiologic activity of the drug, because the active principles are held in solution, and it is easily administered.
In the preparation the drug or drugs - (a) may remain unchanged, as in the emulsion of cod-liver oil, rhubarb pills, or powder of ipecac and opium (Dover's powder); or (b) may be changed by chemic reaction, as in Fowler's solution or Basham's mixture; or (c) may be made to yield their active constituents to a suitable solvent, as in preparations made by extraction. Preparations, too, may be employed in the manufacture of other preparations, as cinnamon water in making chalk mixture, and the extract of belladonna in making a belladonna plaster.
Extraction is the process of obtaining the active constituents of an animal or vegetable drug by means of a suitable solvent. By this process the inert woody fiber, cellulose, and other matters that are insoluble in the solvent employed are left behind, so that only the soluble matters of the crude drug appear in the preparation. In extraction the solvent is known as the menstruum, and this differs with the different drugs or types of preparation. It may be water, alcohol, alcohol and water, alcohol and glycerin, glycerin, wine, acetic acid, ether, chloroform, etc. Official preparations made by extraction are:
A. With aqueous solvent - infusions and decoctions.
B. With alcoholic solvent (in most instances) - extracts, fluidextracts, and tinctures.
C. With wine - wines.
D. With diluted acetic acid - vinegars.
E. With ether - oleoresins.
Preparations made by extraction represent the activity of the crude drug, but in addition to the active principles, always contain more or less physiologically inert matter which has gone into the solution. Such inert matter is known as the "extractive," and it consists of such substances as fat, wax, oil, tannin, chlorophyll, etc. Such "extractive" is mostly colloidal in nature, and has a tendency to retard the absorption and the activity of the active constituents.
There are two types of percentage liquids - the chemic and the pharmaceutic. The chemic percentage liquid deals only with weight, as chemic reactions involve relative weights regardless of volume. To make a 20 per cent. chemic solution, 20 grams of the substance to be dissolved are mixed with 80 grams of solvent; therefore, 100 grams (weighed) of the solution would furnish 20 grams of the contained ingredient. In the pharmaceutic percentage liquid, however, solids are weighed and liquids measured, so that in making a 20 per cent. pharmaceutic solution 20 grams of the substance to be dissolved are mixed with enough solvent to make the total measure 100 c.c. Of such solution, 100 c.c. (measured) will contain 20 grams of the drug. In the practice of medicine, liquid remedies are always administered by measure, for one cannot carry scales to the bedside; therefore the United States Pharmacopoeia adopts the pharmaceutic percentage liquid, so that a given measure will contain an easily calculated amount of each essential ingredient. The volumetric solutions used in chemic analysis are made on the same plan. By this method a very soluble chemical, such as potassium iodide, may be had in 100 per cent. solution.
As an illustrative example of the difference between the chemic and the pharmaceutic percentage liquid, let us take a 10 per cent. solution of cocaine hydrochloride in normal saline. In the pharmaceutic solution, 10 grams of the cocaine salt are dissolved in a quantity of normal saline, and sufficient normal saline added to make the finished solution measure 100 c.c. Of this solution, a measure of 10 c.c. will give 1 gram of the cocaine salt, a measure of 1 c.c. will give 0.1 gram, and there is a simple relation between the measure of the solution and the amount of cocaine it contains. In the chemic solution 10 grams of the cocaine salt are dissolved in 90 grams of the normal saline, so that if one wished to use 0.1 gram of cocaine hydrochloride, one could not get it by measure, since there is no easily calculated relation between the measure of the liquid and the weight of its dissolved constituents; therefore, one would have to weigh off 1 gram of the solution. Such weighing cannot be done in practice, therefore the chemic percentage method is not suitable for liquids for medicinal use.
To conform with the idea of weighing solids and measuring liquids the Pharmacopoeia specifies that in liquid preparations made by extraction a definite weight of the drug shall be employed in making a definite volume of the finished preparation. Hence these preparations have a definite relation in strength to the drug from which they are made, for the active ingredients of a definite weight of the drug are in the solution. The strengths of pharmaceutic preparations are indicated by the amount of drug used in their making, whether the drugs themselves are in the finished preparation or only their extracted constituents. Thus a measure of 100 c.c. of the tincture of digitalis represents the medicinal activity of 10 grams of digitalis leaves; the tincture is, therefore, of 10 per cent. strength. A measure of 100 c.c. of the fluid-extract of cascara represents the medicinal activity of 100 grams of cascara, hence the fluidextract is of 100 per cent. strength.
Pharmaceutic preparations are simple or compound. The simple preparations represent the activity of one drug only; the compound preparations, the activity of more than one drug. For example, rhubarb pills have rhubarb as the only constituent, while compound rhubarb pills contain rhubarb, aloes, myrrh, and oil of peppermint.
The simple preparations are given simply the name of the drug prefixed by the name of the kind of preparation, as: Syrup of ginger (syrupus zingiberis), infusion of digitalis (infusum digitalis). The compound preparations have two types of nomenclature. If the active drugs are only two in number, or in some cases three, all are mentioned in the name, as: Pills of aloes and iron (pilula aloes et ferri), elixir of the phosphates of iron, quinine, and strychnine (elixir ferri, quininae et strych-ninae phosphatum). If the important drugs are several in number, especially if one overshadows the others in importance, only one drug is named, and the name of the class of preparation is modified by the term compound. Examples are: Compound tincture of cinchona (tinctura cinchonae composita), which is made of cinchona, serpentaria, and bitter-orange peel; compound licorice powder (pulvis glycyrrhizae compositus), which contains glycyr-rhiza, senna, and sulphur; and compound rhubarb pills, mentioned above.
A few compound preparations of this kind do not bear a drug name, but the name which indicates their use in medicine, as compound cathartic pills (pilulae catharticae compositae).