This section is from the book "A Text-Book Of Materia Medica And Pharmacy For Medical Students", by Velyien Ewart Henderson. Also available from Amazon: A Text-Book of Materia Medica and Pharmacy for Medical Students.
To Magistral or Extemporaneous Pharmacy belongs the compounding and dispensing of drugs. Its successful performance naturally has to be preceded by a knowledge of their physical and chemical characters. Dexterity in the art can only be secured by long practice, something for which the medical student has no opportunity. The dispenser stands between the prescriber and the patient and only a very intimate acquaintance with the characters and doses of medicines will enable him to successfully perform his duty to each. The physician who dispenses his own remedies assumes a double liability in that he becomes sponsor for the proper selection of the remedy as well as its preparation so that the patient may take the prescribed quantity without danger to himself.
Carelessness in weighing and measuring medicines should not be tolerated on any account. Guessing at the weight or volume of ingredients is criminal.
On the Dispensing of Mixtures. In this procedure as in every other the dispenser must take the greatest care that all the apparatus used must be scrupulously clean. The bottle selected must be of such capacity as to be filled by the ingredients. Sick persons are often so full of fear and doubt that the slightest unusual feature, -insufficient bulk, change in colour, flavour or clearness of a mixture, at once awakens suspicion.
If the prescription presents no incompatibles and the dispenser knows that it will be and remain clear when filled, it may be dispensed directly into the bottle. Or if it is intended that a precipitate is to be produced, the same procedure may usually be followed. It is a good rule to introduce all fluids through a funnel as this serves to keep the neck and sides of the bottle clean and dry. Solutions of common salts and any other solution that is not quite as clear as it ought to be should always be passed through a filter; a little absorbent cotton in the neck of the funnel often serves very well. The fluids of least bulk should as a rule be first dispensed unless there is some special reason for deviating from this, for example very volatile fluids should be dispensed last.
Separate the soluble solids and dissolve them in a portion of the menstruum by trituration in a mortar before placing them in the bottle. Never permit solids that are completely soluble in the vehicle to leave the dispensary undissolved. The method of adding soluble salts directly to the mixture while it may save some time is not to be commended because of the frequency with which their solutions contain foreign matter which requires filtering out. Insoluble dry drugs if prescribed should be reduced to a fine powder, mixed with some of the menstruum and added to the rest. In many instances it is well to suspend insoluble drugs by the addition of gum, mucilage or a viscid fluid such as syrup or glycerin. If the vehicle be water or an aqueous fluid and there are oils, balsams or oleo-resins ordered these should be emulsified before being added to the bottle. The remainder of the menstruum is now added and the bottle corked.
The dispensing of fluid medicines necessitates a more complete acquaintance with the subject of incompatibliity than is the case with any of the other forms of extemporaneous prescriptions.
All solids should be weighed and all fluids measured.
Fluids must be poured from the back of the dispensing bottles so as to save the labels.
Powders must be taken from their bottles with a long spatula.
The physician dispensing his own prescriptions may facilitate his office work by keeping many of the frequently used drugs prepared in concentrated solution. These are made by dissolving a known weight of the drug in a sufficient quantity of the solvent to make a definite volume of the final solution. For instance if eight drachms of Bromide of Potassium are dissolved in the quantity of water required to make a solution measuring four fluid ounces, each four fluid drachms of the latter will then contain one drachm by weight of the Bromide. These are called dispensing solutions and are quite different to the percentage solutions of the chemical laboratory and to those of the Pharmacopaeia. Such salts as the Bromides, Iron Tartarate, Magnesium Sulphate, Potassium Iodide, Chlorate and Nitrate, may readily be kept in this form; Bicarbonates in solution are liable to change as do many of the organic preparations such as Chloral. Dispensing solutions should always be kept in the dark. Though the result is hardly so good as when the Pharmacopaeial method is used, flavoured syrups may often be prepared by adding to a Simple Syrup a liquor of the flavour required. In many other cases concentrated liquors or extracts may be obtained from the pharmaceutical houses which may be diluted so as to approximate the official preparations. In some cases it is of advantage to keep weighed out and wrapped as powders, the insoluble drugs in powder form needed for such a mixture as Mistura Cretæ. Infusions especially of digitalis should always be freshly made.
Percentage Solutions. The disadvantages of the Imperial System are most clearly seen when dispensing this class of solutions. Absolute exactitude cannot be attained. For dilute solutions (1-3%) of highly active drugs, e.g. Strychnine, Atropine, the following method is to be recommended; - 110 minims of water weigh 100 grains; therefore if a 2% solution be required 2 grains are dissolved in 110 minims. The bulk of the resulting solution is slightly greater than 110 min. and its weight exceeds
100 grains. but for all practical purposes 1 min. will contain 2/100 grain. If one ounce were required, the simplest procedure is to make 4 1/2 x 110 or 495 min. and throw away 15. When a more concentrated solution e.g. an ounce of a 10% solution of a salt, is prescribed a somewhat more complex procedure may best be followed, as it is impossible to estimate what the specific gravity of the resulting fluid will be. The following procedure is often pursued: - There are 437 grains to the fluid ounce, 10% of this is 43. If 43 gr. salt were dissolved in 394 min. the bulk would not be 1 fl. oz., but it 50 gr salt were dissolved in 450 of water the bulk would obviously exceed a fluid ounce and in consequence such a solution might be made and the excess thrown away. Very frequently and with sufficient accuracy for many physician's purposes, percentage solutions are not made as above by weight but by weighing the solids and measuring the liquids, one minim being then considered as weighing one grain; one ounce, one fluid ounce.