Idiosyncrasy. - Before finally deciding, however, on certain drugs, idiosyncracy must not be forgotten; that is, the peculiar susceptibility of some individuals to the action of particular medicines, such as opium, mercury, quinine, essential oils, and ipecacuanha. In almost every instance such idiosyncrasy means increased susceptibility; unpleasant or even dangerous results following an ordinary or even minute dose. It is well, therefore, before ordering such drugs, to enquire whether the patient has taken them previously, and if not, to use them cautiously at first.

2. Selection Of The Preparation

Selection Of The Preparation. The drug having been selected, the particular preparation of it will be selected in accordance with the considerations discussed under the head of varieties of preparations. The Pharmacopoeia affords abundant choice, according to the channel by which it is to be administered. This naturally leads us to consider the Modes Of Administration Of Drugs

(a) By the skin, or mucous membrane continuous with the skin, whether simply applied or rubbed in (liniment, ointment); painted on (pigment); worn on the skin (as a plaster); applied in a state of fine division by fumigation, with or without sweating; used as a gargle, injection, or wash; or insufflated on to a part. The effect desired is usually local only, but it may be general, many drugs being absorbed by the skin.

(b) By the mouth, to act locally on the alimentary canal, and to be absorbed from it, especially from the stomach.

(c) By the rectum, (or vagina in the female), in the form of enema or injection (fluid), or of a suppository (solid). Sometimes drugs cannot be administered by the mouth, either on account of some physical obstacle, repugnance on the part of the patient, or irritability of the stomach; or to spare the strength generally, and the stomach especially, in conditions of exhaustion. Again, the action desired may be a local one on the rectum and pelvic organs, e.g. to relieve pain, destroy worms, or soften retained faeces.

(d) By injection under the skin - subcutaneous or hypodermic injection, or into the tissues - interstitial injection: excellent methods of admitting some remedies into the system with certainty and despatch, and in small bulk.

(e) By application to wounds or diseased surfaces, as lotions, poultices, gargles, injections, collyria; or by the endermic method, i.e. by being sprinkled on a blistered surface.

(f) By inhalation, the substances being sometimes volatile, and intended either to enter the blood through the pulmonary capillaries, e.g. chloroform, or to act directly on the parts to which they gain access in the form of smoke, e.g. cigarettes, powders, etc.; sometimes medicated watery vapours, such as Vapor Conii.

(g) By intravenous injection, very rarely practised in man.

3. The Dose

The Dose. The Pharmacopoeia indicates the limits of ordinary doses, the minimum being the smallest useful dose which it may be wise to begin with, and the maximum being the largest usually given without special reason and caution. Experience alone can teach the practitioner how far he may safely and wisely depart from these limits, to which he is in no wise tied by law. Several modifying circumstances which are to be taken into account with respect to doses must here be carefully noted.

(a) Many drugs have different actions in different doses, which must be arranged accordingly; e.g. anti-monium tartaratum, alcohol, opium, and rhubarb.

(6) The dose must vary with the age of the patient, children getting but a fraction of a dose for an adult. A convenient method of calculating the doses for children under twelve, is to divide the age in years by the age in years + 12, and to use the result as the proper fraction of an adult dose. Thus, for a child of four years the dose will be 4/4+12 = 4/16= 1/4 of an adult dose: for a child of twelve, 12/12+12 = 12/24 = 1/2

Above twelve, and under twenty-one, the dose must lie between 1/2 and a full dose. Delicate persons and patients exhausted by disease resemble children in bearing but small doses.

(c) In particular diseases the ordinary dose may have to be modified. In disease of the kidneys, where excretion is diminished, drugs which are discharged by this channel, such as morphia, are retained in the blood for a longer time, i.e. in larger quantity at any given time after administration, and symptoms of poisoning very readily supervene. Quite a different matter is the effect of a disease in neutralising the effect of a drug given to combat it. Thus, large doses of morphia will be tolerated in severe pain, because the action of the morphia is spent in overcoming the pain. The periods of menstruation, pregnancy, and lactation also require to be considered in prescribing.

4. Frequency

Frequency. Medicines are ordered to be taken one or more times, according to the desired end. Thus purgatives are generally taken in a single dose; an emetic is to be taken once, and repeated only in case vomiting is not induced; whilst tonics are generally ordered three times a day continuously.

5. Duration

Duration. The period for which a drug may be given depends entirely on a variety of circumstances which need not be discussed here. We must refer, however, to accumulation, toleration, custom, and habit. When a drug is allowed to enter the system at short intervals, for a sufficient period, more rapidly than it can be excreted, a time will obviously come when it will have accumulated so much in the tissues as to produce its effects in a marked degree. Powerful drugs, e.g. strychnia and digitalis, may thus begin to act as poisons after having been given in the same doses with benefit .for weeks. On the other hand, certain drugs lose their effect when given for a length of time, from some cause still obscure, e.g. opium. The dose must then be steadily increased, toleration being said to be established by custom. If a patient become dependent on a drug, crave for it, and indulge in it to an unfortunate or even vicious extent, he is said to have developed a habit for that drug, such as the opium and alcohol habits or the habitual use of enemata.