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.
Administration by the mouth. For remote action drugs are most commonly given by the mouth though as pointed out above they may be administered by the rectum, the skin, or the lungs. The use of hypodermic intramuscular and intravenous injections is increasing. When the physician has decided to give the drugs required by the mouth, several pharmaceutical forms may be used; mixtures are still the most commonly employed, though pills are extensively used when the drugs are unpleasant and the dose is small; powders are used only when one wishes to administer larger doses of tasteless or not markedly unpleasant drugs in quantities larger than can be given in pill form; caches and capsules are forms steadily gaining in vogue especially for the administration of drugs having an un pleasant flavor.
Mixtures. It is good practice for the physicain when he writes a prescription for a mixture to use only such drugs as will dissolve and produce a clear solution. This is a good but by no means an absolute rule and indeed we find in the pharmacopoeia very striking deviations from it,e.g. Mistura Ferri. This intentionally contains the incompatibles Iron Sulphate and Potassium Carbonate resulting in the formation of an insoluble precipitate and a murky solution. The Iron Carbonate formed is, however, less irritant to the stomach than the Sulphate. Mixtures containing a precipitate were very commonly prescribed in the past, but today a physician is compelled to pay more attention to the likes and whims of his patients, all of whom have seen and tasted attractive and pleasant patented preparations. In writing a prescription for a mixture the physician should, as a rule, use fluid preparations of the drug selected if such are contained in the Pharmacopoeia. The reasons for this are easily seen if one considers the matter from the view-point of the dispenser. Suppose that thirty doses of Strychnine 1/60 of a grain and Arsenious Anhydride 1/40 gr. are to be given. This would force the dispenser, if the solids were prescribed, to weigh out 1/2 and 30/40 gr. of the two drugs respectively, while if Liquor Arsenicalis 80 min. and Liquor StrychniŠ Hydrochloridi 110 min. were prescribed the same amount of each drug would have been given and the dispenser's work made easy and more rapid as Arsenious Anhydride is difficult to get into solution. The Liquors, Spirits, Tinctures, Liquid Extracts, Waters, Syrups, are the important fluid preparations intended for administration in mixtures. When there is a choice of salts the more soluble one should be used.
The physician must take every possible care that his mixtures are as palatable and as pleasant to the eye as possible. The only colorings provided in the Pharmacopaeia are Crocus, Cochineal, and Red Sanders Wood, and the Compound Tincture of Cardamons Neither Crocus nor Cochineal should be used in an acid medium. All of the above produce shades of red. Sweetening may be provided in the form of Simple Syrup or even better as one of the flavored syrups e.g. Aromatic Syrup, Syrup of Orange, of Ginger or of Tolu. Liquorice contains a particularly sweet flavoring principle and is very greatly used. Amongst the Waters and Spirits the attention of the student may be drawn to the usefulness of Chloroform, Cinnamon, Orange, and Peppermint. Rose flavors are more usually used today for lotions and ointments than for mixtures. Acids and Bitters are also in many cases useful flavors. General rules for flavoring are extremely difficult to give and since the taste of each physician and indeed of each of his patient will vary, it is difficult even to give useful hints. For such vegetable drugs as Digitalis, Ergot, Ipecac, Krameria, and the bitter of most alkaloids Syrup of Orange is one of the best flavors, aided perhaps by some water such as that of Cinnamon or Peppermint; for Opium, Ginges forms a good covering: for the salts of Potassium, such flavoring Waters ar Chloroform or Peppermint with Aromatic Syrup may be used; Potassium Iodide and Quinine may be covered with Extract of Liquorice; Sodium Salicylate by Cinnamon Water, and Syrup; Copaiba by Peppermint.
Pills form perhaps the best method of administering unpleasant drugs whose dose is small. As many people find pills difficult to swallow, they must be made as small as possible, and should never exceed 5 grains in weight and rarely should exceed 3 grains. In consequence of this the student should examine with care the preparations of any drug which he intends to give in pill form and choose the one with the smallest dose. The only exception to this rule should be made when one of the preparations has physical properties which would be of value in forming a pill mass. The prescriber must also bear in mind the fact that unless amongst the drugs that he wishes to prescribe there is one whose physical properties are such as to bind the others together some adhesive substance or excipient must be added by the dispenser and that this will necessarily increase the bulk of these pills to a certain extent. The best excipient for general use is probably Tragacanth either in the form of its Glycerin or CompoundPowder, as very small quantities of these are needed. Powdered Hard Soap may be used with powdered vegetable drugs and gum resins, and Curd Soap with essential oils and creosote. As each pharmacist is apt to become familiar with one particular type of excipient and prefer it to all others it is often well to omit the excipient from his prescription, but he must not fail to remember that it must be added and will increase its bulk.
The student will notice that the solid Extracts are in many cases the most compact form in which vegetable drugs can be given and they and the Green Extracts, which are very sticky and form good pill bases, are introduced into the Pharmacopoeia as ingredients of pills. It is rarely that aqueous or alcoholic solutions can be incorporated in pills.
Powders. A physician, when considering the administration of drugs in powder form, must always carefully consider the flavor of the principal drug and whether if it is unpleasant or even tasteless, its palatibility can be increased by adding some flavoring. It is rarely that the taste of a disagreeable powder can be successfully covered. If a disguising flavor is wanted, Sugar, Liquorice and Cinnamon are perhaps amongst the best. The physician must remember that deliquescent salts cannot be given in powder form.
Cachets and Capsules. These are much used in modern dispensing as they enable the physician to administer disagreeable and bulky powders and also oils in an elegant manner. Roughly speaking one may order up to 10 grains of a drug in capsule form, and up to 20 in a cachet. If the drugs are very heavy these quantities may be readily increased. Fluids, with the exception of oils, should not as a rule be given in capsules.
Administration of drugs by hypodermic, intramuscular, and intravenous injections. These methods of administration make certain the complete absorption of the drugs given if they are soluble in the fluids of the body. And in consequence of this much smaller quantities are used than for administration per os. Drugs given by hypodermic injection should be non-irritant to the sensory nerve-endings of the part. They should in consequence be neither acid nor alkaline. When given intramuscularly this point should also be observed, but when given intravenously it becomes of very little importance as the drug does not come in contact with the sensory endings. If drugs are given hypodermically, roughly one half the dose that would be given per os may be used. If given intramuscularly, a slightly smaller quantity is usually given and if given intravenously, owing to the rapidity with which it reaches the point of attack, approximately one-tenth that would be given per os is administered. These rules are by no means absolute as drugs differ so greatly in their physical and pharmacological properties. In the case of all these methods of administration the very greatest care must be taken that the solutions are strictly aseptic and that the patient's skin is carefully sterilized before the administration is undertaken.