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.
As each patient must be considered as an individual and given separate care and thought, it is impossible in such a book as this, which does not treat of therapeutics, to do more than indicate some of the points which must be understood and borne in mind when about to write a prescription.
Drugs are administered by the physician either for a local effect, that is, produced in the immediate neighborhood of the point of application, or for their remote effect, that is, produced generally on various organs throughout the body, or on some special organ remote from the point of application for which they have a special affinity.
It is consequently essential for the prescriber to decide in the first place what organs he wishes to influence and whether they are such as he can reach by local application or not. Then in the second place, he must decide what drugs he desires to use to affect these organs.
For Application to the Skin. Very few drugs can be absorbed into the system through the skin with such rapidity that they accumulate within the body sufficiently to be used by the physician for their remote pharmacological action. The outstanding example of those that do so is mercury. Most other drugs are excreted by the kidney and bowels so much more readily than they are absorbed from the skin that no remote action can be expected.
The rapidity with which drugs may penetrate the skin can be greatly increased by dissolving them in some solvent which penetrates more readily than they do themselves. Such solvents are used as the bases of those ointments from which we wish an action after absorption. Alcohol, to a slight extent, olive-oil, wool-fat, and lard, are all absorbed by the skin. Olive-oil is perhaps the best base to aid absorption. Wool-fat is very nearly as good, while lard is not very efficient. The paraffins, resins, and soaps, are scarcely absorbed at all, and solution in them rather delays than furthers the absorption of substances made into ointments or plasters with them.
A local effect, but one produced in the skin after absorption, is naturally more easily obtained. Oil of Mustard developed in a mustard poultice or plaster, - Cantharidin from its Collodion, Liquor, Emplastrum,Unguentum, or Tincture, - Croton Oil, Turpentine, Ammonia and Chloroform, readily pass through the skin and by reflex stimulation set up a more or less marked local inflammation and possibly a remote reflex effect. The student will note that the bases of Unguentum Cantharidis are more soluble in the skin than those of the Emplastrum and consequently contain a lower percentage of Cantharis. Atropine, morphine, cocaine, camphor, potassium iodide and salicylates all pass through the skin in quantities sufficient to produce local deep effects if they are combined with suitable bases such as the oils, wool-fat, and alcohol, and especially if they are aided by heat or rubbing or both. Such bodies as ammonia, chloroform, camphor, and turpentine, which by a local action after absorption lead to a dilatation of the vessels of the skin, aid in the more rapid absorption of such other bodies as can penetrate the skin, and this forms one reason for their inclusion in ointments, liniments, plasters. If the student, with these ideas in mind, will examine the pharmacopceial liniments and ointments he will gain a good idea of how he may perscribe for local and reflex effects by way of the skin.
Lead Plaster is largely used for mechanical and supporting purposes, but none of the other pharmacopceial plasters save those of Cantharis and possibly Belladonna are frequently employed.
An antiseptic in the form of boracic acid 5%, salicylic acid 1/4%, or benzoic acid 1%, is frequently added to dusting powders consisting of talc, starch, or zinc oxide, or to ointments containing some such inert protective powder, (e.g. Unguentum Zinci Oxidi).
In many cases a purely superficial effect on the skin is desired. Antiseptics must often be applied to the skin in order to kill pediculi or disease germs which are lying upon it. For this purpose phenol in 1/2 - 2% watery solution, mercuric chloride 1-500-2000, boracic acid in a saturated solution, are amongst the common drugs used. If the skin be abraded, very much weaker solutions of phenol or mercury must be applied, otherwise sufficient antiseptic would be absorbed to produce a remote effect, or they may be replaced by less readily absorbed antiseptics such as iodoform, ichthyol, thymol, or resorsin. In any such case, one of the above drugs may be applied in the form of an ointment whose base is not readily absorbed by the skin. A very considerable insight into the character of preparations intended for external use may be gained by the examination of the examples of liniments and ointments and of the preparations of mercury. If the constituents and bases of the following preparations of mercury are examined it may be seen that the Emplastrum Hydrargyri, Emplastrum Ammoniaci cum Hydrargyro, Unguentum Hydrargyri Oxidi Flavi, Unguentum Hydrargyri Oxidi Rubri, Unguentum Hydrargyri Ammoniati, and the Lotio Hydrargyri Flava vel Nigra would be relatively slowly absorbed; the Unguentum Hydrargyri Compositum, Unguentum Hydrargyri Iodidi Rubri, Unguentum Hydrargyri Oleatis, Unguentum Hydrargyri Subchloridi, more readily; while the Unguentum Hydrargyri Nitratis and Linimentum Hydrargyri will be still more readily taken up. The first group of these would therefore be more generally used for purely external and local purposes where they would act as antiseptics. The last group might readily be used for remote and specific effects.
From the inlets of the body covered with mucous membranes, drug-stuffs are much more readily absorbed than from the skin. Drugs applied to them can readily be made to exert a remote action.
For application to the rectum. Enemata are sometimes used to produce remote action. This method is only resorted to when the drugs to be given are either too unpalatable to be taken per as or when they would irritate the mouth or stomach of the patient. Drugs given per rectum are usually administered in, roughly, twice the dose in which they would be taken by the mouth. If the drug-stuffs contained in an enema are to absorbed it is necessary that they should not be irritant to the mucous membrane nor should its bulk be so large as to mechanically set up movements. Hence watery or weak alcoholic mixtures 1-2 ounces in bulk are usually used. Medicaments which are very readily absorbed may be given in the form of a suppository. Three of the pharmacopaeial suppositories contain drugs which exert remote actions. These are, Suppositoria Belladonnę, Morphinę, Plumbi.