This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
(a) Into the blood-vessels by injection. This method is rarely used in man, excepting for transfusion of physiological saline solution (a teaspoonful of common salt to the pint of sterilized water at the temperature of the body) in cases of great loss of blood.
(b) Into the subcutaneous tissues by hypodermatic injection. The skin of the patient, where it is lax, should be raised between the thumb and forefinger of the operator's left hand; the skin of the external surface of the forearm is often selected. In his right hand he takes a perfectly clean syringe containing the quantity of fluid to be injected, and fitted with an aseptic, hollow, silver needle, which is thrust under the raised piece of skin, but not into the muscles, for about an inch, care being taken to avoid wounding a vein. The syringe is slowly emptied, then withdrawn, and the thumb pressed lightly upon the seat of injection for a few seconds. The advantage of this method is that the drug is surely and quickly absorbed. The fluid used must not contain solid particles, nor be irritating, or abscesses will result; it must be aseptic, and therefore, if it is not freshly prepared, it may contain a little carbolic acid - or, better still, boric acid, for this is non-poisonous and non-irritating. The bulk injected should, if possible, be about five minims; .30 c.c.. For injections that are not in constant use it is advisable to keep the drugs in the form of soluble tablets or lamellae, and to dissolve one in a few minims of water as required.
(c) Into serous cavities by injection. This method is rarely used in man except when the object is antiseptically to wash out a serous cavity, as the pleura which has been opened, or to produce adhesive inflammation, as in the injection of irritants into the tunica vaginalis.
Drugs are most frequently given by the mouth, to be absorbed from the mucous membrane of the stomach or intestines, but the rate of absorption is much slower than from the subcutaneous tissue, and will depend upon whether the drug is readily soluble in the gastro-intestinal secretions, and whether it is given on an empty stomach, in which case it will be quickly absorbed; or on a full one, when it will be slowly absorbed.
When it is intended that the drug shall act only in the intestine, pills, made purposely insoluble in the gastric fluids, are administered. Some drugs, given by the mouth and absorbed from the stomach, probably never reach the general circulation, as they are excreted in the bile by the liver. The drug must be in a pleasant, palatable form, and generally so combined as not to irritate.
Drugs are sometimes given by the rectum - in a solid form as suppositories, in a liquid form as enemata or clysters; but they are not dissolved nor absorbed here so quickly as in the upper part of the gastro-intestinal canal.
For local effects they may be given by the urethra or vagina (injections, bougies, pessaries), or by the respiratory passages (inhalations, cigarettes, sprays or nebulae for inhalations; insufflations for blowing into the throat and larynx; pigmenta, gargarismata, trochisci, for a local effect on the mouth and pharynx; nasal douches for the nose). For sprays an atomizer is required. Sometimes volatile drugs, as chloroform or amyl nitrite, are inhaled for their general effect.
Some drugs may be absorbed from the skin if mixed with some fatty substance, especially hydrous wool fat. In this way mercury may be absorbed by being rubbed in; but drugs are chiefly applied to the skin as ointments, plasters, etc., for their local effect.
Some drugs may be absorbed from the skin when they are volatilized. In this way mercury is introduced into the system by fumigation.
They are also applied to the eye and ear as drops and washes.