It must be apparent that the ordinary average adult dose is not the dose for every one under all circumstances. Some of the factors modifying the dose are:

1. Body Weight

In pharmacologic experimentation it is customary to estimate the dosage in proportion to the weight of the animal. Within certain limits this should be a good method with humans, and it is the basis of Clark's rule, which assumes that the average weight of an adult is 150 pounds. The rule is -

Adult dose X weight/150. But a patient in bed cannot be weighed, and it takes an expert to guess such a one's weight correctly; and a man with dropsy or an adipose patient would have some extraneous weight to be allowed for. So, as a matter of fact, either on account of our highly organized nervous systems or on account of our ways of eating and drinking and working, or for other reasons, the rule of weight does not seem suitable for practical use.

2. The Age

It is evident that the dose for an adult is not the same as that for a child. Yet to establish a working rule is not easy, for not only is there no regular increase in a child's weight according to age, but there is also unequal development of the different systems of the body. The weight rule would be the best but for its difficulty of adoption, and to multiply the adult dose by a simple fraction with the child's age as numerator and the supposed earliest adult age as denominator, will not be correct. It will not do, for example, to take an arbitrary age of twenty or twenty-four as the adult age, and take one-twentieth or one twenty-fourth for each year of the child's age. The following table of the average weights at the different ages, taken from Bowditch's statistics in 8008 children in Boston, and Paster's of 14,744 children in St. Louis, as recorded by Holt, shows how absurd it is to estimate the dose at two years as twice that at one year, etc. The figures given are for the boys, those for the girls being for the most part not more than one to three pounds different.

Age

Weight

Half year ........................................................................................

16.0

pounds

One year........................................................................................

21.0

"

Two years........................................................................................

27.0

"

Three years........................................................................................

32.0

"

Four years........................................................................................

36.0

"

Five years........................................................................................

41.2

"

Six years........................................................................................

444

"

Seven years.....................................................................................

48.6

"

Eight years........................................................................................

53.5

"

Nine years........................................................................................

58.7

"

Ten years........................................................................................

64.6

"

Eleven years....................................................................................

70.6

"

Twelve years....................................................................................

76.7

"

Thirteen years...................................................................................

83.7

"

Fourteen years.................................................................................

94.0

"

Fifteen years....................................................................................

107.3

"

Sixteen years....................................................................................

119.1

"

From these figures a fairly accurate age-weight rule would be: age+3 / 30 X adult dose. In other words, in writing for 30 doses

(4 ounces with 1 dram dose) put down as many minims or grains as the age + 3; in writing for 15 doses (2 ounces with 1 dram dose) put down half as many minims or grains as the age + 3. In the metric system put down: the adult dose X (age + 3) X 3, and move the decimal point two places to the left. Two other rules in common use are Young's and Cowling's:

Young's rule is:

Adult dose

X

age

age +12

Cowling's rule is:

Adult dose

X

age at next birthday

24

In prescribing by this rule, all that is necessary is to write for 24 doses and set down for each ingredient the adult dose multiplied by the age at next birthday.

Fried's rule for infants under one year is: Adult dose X age in months / 150.

In some cases these rules do not apply, e. g., children react strongly to opium and other narcotics, while, on the contrary, the child's dose of a cathartic or belladonna or arsenic approaches that of an adult. We have seen the same amount of belladonna given to a father and to his son six years of age with equal effect; and a child of three years not one whit more affected by a grain of calomel than was her mother by half the dose. On the other hand, we have seen a child of one year "doped" by one-twentieth of a grain of powdered opium.

In old age the dose must be, as a rule, somewhat less than in the prime of life; and especially must skin irritants, irritant cathartics, narcotics, and depressant drugs be used with caution.

3. Sex

Women usually require smaller doses than men, not only because of their average smaller stature and quieter life, but also because of their greater susceptibility to any influences. During menstruation and pregnancy irritant cathartics, and during lactation saline cathartics, are to be avoided or used with caution.

4. Temperament, Race, Occupation

The patient of highly neurotic temperament is more susceptible than the phlegmatic person. Such difference may be racial, the excitable Italian, for example, being more easily affected than the stolid Swede; or it may have to do with activity and occupation, the athlete or the man who works all day out-of-doors and is inured to hardship being less readily affected than the man of sedentary habits, the merchant, student, or artist.

5. Previous Habits (Toleration)

The morphine habitue can take with impunity a dose of morphine large enough to poison one not habituated, and will obtain no effect from the ordinary dose. An old toper with cirrhosis of the liver will fail to get a medicinal effect from the usual dose of a tablespoonful of whisky.

6. Idiosyncrasy And Susceptibility

Idiosyncrasy is that condition in which a patient develops special and unusual effects from a remedy or food. Some people develop a rash after eating strawberries, others after eating lobster, fish, or buckwheat. Sometimes all the members of a family show such an idiosyncrasy to some special article of food, and it is manifest in successive generations. The same is true of drugs. A minute amount of cocaine dropped in the eye or applied to the nasal mucous membrane may cause dangerous symptoms in one patient, though cocaine is used in the eyes and noses of thousands of other patients without any untoward symptoms at all; or a dose of anti-pyrine may be followed by a marked rash, which recurs each time the drug is taken. These are unusual and unexpected effects, and depend not so much on the size of the dose as upon a specific and unusual hypersusceptibility of the patient toward the drug.

An ordinary increase of susceptibility means lowered resistance - a condition in which the usual or expected effects are produced by less than the usual amounts. For example, two or three grains of quinine sulphate produce in some people the ringing in the ears, deafness, and headache that in most persons do not come from less than 10 or 20 grains. Diminished susceptibility means heightened resistance, the patient showing the usual effects, but only after larger doses than usual. For example, some persons can take two or three cups of coffee and then sleep soundly, though this is enough to keep the average person wide awake for hours.