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..
The study of doses is termed Posology. In determining the dose the following considerations have to be borne in mind:
1. Age. - The adult dose is that for a person between twenty and sixty years old. For women the dose should be somewhat smaller than for men.
For children under twelve, add twelve to the age, and divide the age by the number thus obtained. Thus, for a child aged eight the dose will be 8 / 8 + 12 = 2/5 of an adult dose. From twelve to sixteen years from 2/3 to 1/2 the adult dose is required, and from seventeen to twenty years from 4/5 to 2/3. There are exceptions to this rule for individual drugs; e.g., children take iron, cod liver oil, arsenic, and chloral hydrate very well, but they can take only very small doses of opium. Cowling's rule - divide age at next birthday by twenty-four - requires rather less calculation and is generally of sufficient exactness.
Above sixty years of age the dose should slightly diminish as age increases.
2. Weight. - In pharmacological experiments the dose should always be expressed as a proportion of the weight of the animal. In man the weight is not often considered, for it depends so much upon bone and fat, which are not active tissues.
3. Habit. - A man who is constantly under the action of a drug becomes very insusceptible to it. Thus an opium-eater requires enormous doses of opium to produce any effect. A person who habitually takes purgatives requires very strong ones to open the bowels.
4. Idiosyncrasy. - The susceptibility to drugs varies very much. Some persons are salivated by minute doses of mercury, others bear it very well, and there is hardly a drug to which some people are not exceptionally indifferent or susceptible.
5. Time of Administration. - Drugs all act to greatest advantage when given so that their effect will be produced at its natural time. Thus soporifics act best when given in the evening, slowly acting purgatives when given overnight, quickly acting ones when given before breakfast, ergot when given during labor. Drugs which are readily decomposed by the contents of the stomach should be given when that viscus is empty, preferably a half hour before the meal time.
6. Mode of Administration. - We have seen that drugs are rapidly absorbed from the subcutaneous tissues. Therefore a smaller dose is required for subcutaneous injection than when the same drug is given by the stomach, for absorption is slow from the upper gastro-intestinal tract. It is slower still from the rectum therefore, to produce effects more immediately, the dose must be larger. Also certain drugs are excreted by the liver or destroyed in it when given by the stomach. Absorption takes place quickly from an empty, slowly from a full stomach.
7. Mental Emotion. - Sometimes if the patient's mind is particularly fixed on the action of the drug, a small dose is powerful. For example, often if the patient is convinced he will sleep, a very small dose of morphine is all that may be required.
8. Temperature. - As the action of the drug on the organism is often partly chemical, the temperature must, in cold-blooded animals and excised structures, as muscle, etc., help to determine its action; but the temperature of man varies within so few degrees that this is not an important factor in medicine.
9. Preparation of Drug. - A smaller dose of a soluble preparation, as a tincture, will be required than of a solid preparation, as a pill, which will have to be slowly dissolved before absorption although in the latter case much depends upon the process of manufacture.
10. Rate of Excretion. - It is obvious that, other things being equal, for prompt action a smaller dose will be required of a drug that is rapidly excreted than of one which is slowly excreted. It is also true that, in order to maintain a continuous effect from drugs which are rapidly excreted, the doses must be repeated at shorter intervals.
11. Cumulative Action. - Sometimes it is found that if a person has been taking a drug regularly, but without the production of any poisonous symptoms, these will suddenly develop. This is said to be due to the cumulative action of the drug. It may be caused by the following circumstances:
(a) The drug may be absorbed more rapidly than it is excreted. This is the cause of the cumulative action of mercury and lead, both of which are excreted with difficulty by the kidney.
(b) There may be a sudden arrest in the excretion of the drug. It is probable that digitalis and strychnine, when the quantity of them in the tissues reaches a certain amount, contract the renal vessels, and hence excretion is arrested.
(c) It is possible that, owing to an alteration in the intestinal contents, a drug which was previously very slowly dissolved becomes quickly dissolved, and hence rapidly absorbed.
12. Disease. - The physiological action of drugs, and consequently the dose, are profoundly modified by disease. For example, a patient with peritonitis will bear enormous doses of opium. Antipyretics, which do not affect normal temperature, powerfully depress a febrile temperature.
The tendency of modern therapeutics is towards smaller and more frequently repeated doses.