This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
When we say the dose of a drug, we mean the therapeutic dose for an adult, i. e., the amount ordinarily required to produce a medicinal effect. The Pharmacopoeia gives the average therapeutic dose, and for convenience this is the dose to learn, in most instances.
The minimum dose is the smallest capable of producing a medicinal effect - not quite so small, however, as two drops of the ninth dilution of the homeopaths, which Oliver Wendell Holmes estimated to be of the strength of one drop in ten billion gallons. A maximum dose is the greatest dose that can be administered without probability of poisonous effects. A toxic dose is a poisonous dose.
Remedies are administered either in single doses or in repeated doses. A single dose of a medicine may be given all at once, as two compound cathartic pills or an ounce of whisky; or in divided doses, as when one grain of calomel is given in one-quarter grain tablets, one every half-hour for four doses.
Repeated doses may be intended to have an effect just at the time of administration, as a bitter before each meal to improve the appetite; or to have a continuous effect, as digitalis for a disordered heart. To produce a continuous effect, remedies are usually given three or four times a day, for, as a rule, it is too great trouble for patients to take medicine more often than this. Even very sick patients should not be disturbed by too frequent medication.
Sometimes a powerful drug given for continuous effect is administered in too large amounts for ready secretion, so that it accumulates in the system until poisonous symptoms appear. Such a drug is known as a cumulative poison. The ill effects are dependent upon the failure of elimination to keep pace with the ingestion of the drug. The most common drugs to give cumulative effects are digitalis, arsenic, mercury, and lead. Lead and arsenic, indeed, are so slowly excreted that they may accumulate in the system even when taken only in the minutest quantities at a time, as from drinking-water that has lain in leaden pipes, or breathing the air of a room with an arsenic color in the wallpaper.
The phrase "pushing a drug to its physiologic limit" is sometimes employed when a remedy is given in gradually increasing doses until toxic symptoms begin to appear.