This section is from the book "Practical Materia Medica And Prescription Writing", by Oscar W. Bethea. Also available from Amazon: Practical Materia Medica and Prescription Writing.

By the dose of a drug is meant the average dose for the average adult under average conditions. Variations from the usual require special consideration.

The prescriber must consider the age, size, sex, temperament, habits, and condition of the patient, and the action of the drug employed.

Young's rule is the one most commonly used for proportioning the dose for a child. It is: Divide the age of the child in years by the age plus twelve to obtain the fraction of the adult dose. For example: If the adult dose of a medicine is 20 grains, to find the dose for a child four years old, divide the age (4) by the age (4) plus 12 to obtain the fraction of 20 grains desired:

Five grains would, therefore, be the dose for a child four years old.. Another rule is: Make 20 the denominator of a fraction the numerator of which is the age of the child expressed in years. The result is the fraction of the adult dose. By this method if the dose

4 / 4 + 12 = 4 / 16 = 1/4. 1/4 of 20 = 5 for an adult is 20 grains, to find the dose for a child four years old the following would be the calculation:

4 / 20 = 1/5. 1/5 of 20 = 4.

Four grains would, therefore, be the dose required.

It will be observed that these rules only apply to those children whose ages are expressed in years. In the case of infants each is a problem unto itself.

The size of a patient is naturally an important factor. It would be manifestly unwise to expect a child four years old and weighing only 20 pounds to tolerate what would be a normal dose for another child of the same age and weighing 40 pounds. A man six feet high and weighing two hundred pounds may require different dosage from one five feet high and weighing one hundred pounds.

Women are, on the average, smaller than men; their organs are smaller; they have less blood; they are weaker and perform less work. These facts should receive due consideration in prescribing, and the average dose is usually smaller than for men. There are some exceptions; for example, purgatives, which are generally required in larger doses and stimulants in comparatively smaller doses for women than for men.

This in some instances is an important factor. For example: A patient of a highly nervous type can tolerate less strychnine than can one of a phlegmatic temperament.

This must often be considered. One addicted to opium, alcohol, coffee, etc., will require larger doses of these drugs to produce a desired effect than will one who has not previously used them. A patient who has been taking potassium iodide can frequently be given comparatively large doses without unpleasant effect.

The condition of the patient is almost always an important consideration. The weak, anemic patient may not be able to tolerate the same dose of a purgative that would be a benefit to one who was of the robust, full-blooded type. A child in the paroxysmal stage of whooping-cough may be given antipyrine to advantage in doses several times the usual proportion.

Some drugs are exceptions to the foregoing rules. For example: The same dose of castor oil is usually given to a child over two or three years old as to an adult. Calomel is ordinarily given in comparatively large doses to children, while opium is usually best administered in comparatively small amounts to children.

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