The art of writing good prescriptions presupposes a knowledge of the antagonistic action of drugs, and a working, practical knowledge of ordinary chemical reactions. The administration of two drugs whose physiologic action is antagonistic, such as the coincident administration of strychnine and a bromide, or of digitalis and aconite, represents therapeutic incompatibility. However, a physician skilled in the use of drugs may modify an unpleasant action of a drug by giving coincidently another drug that counteracts such action, as, for instance, a bromide will control the unpleasant head symptoms caused by quinine. Bromide is a sedative to the central nervous system, and quinine is a stimulant to the central nervous system. Or, atropine may be given with morphine to antagonize the sedative action of the latter on the brain and respiratory center. Though certain kinds of therapeutic incompatibilities are advisable, others are inexcusable for instance, it is inexcusable to give a quinine and strychnine tonic after the evening meal, or to allow tea or coffee at the evening meal, and then to give a hypnotic at bedtime because the patient cannot sleep. Chemical incompatibility is closely related to pharmacal incompatibility, and the latter can be avoided only by writing simple prescriptions and studying new combinations with the help and advice of a pharmacist. Many of the United States Pharmacopoeial and National Formulary preparations are so carefully chemically balanced as to be thrown out of balance by the addition of most anything, and often even by more or less dilution; the change occurring may be precipitation, effervescence, or the formation of new chemical compounds. Gross chemical mistakes should not occur in prescription writing. To avoid such errors, the following brief list of "do" and "don'ts" should be memorized. The chemical reason for such advice is not given, as reference to books on chemistry will elaborately outline the reactions. Prescribe alone: Acids, unless very dilute. Alkalies and alkaline carbonates in solution. Alkaloidal salts. Arsenic in solution. Ferric and ferrous salts. Iodides. Mercuric chloride. Tannic acid, tannates, and preparations containing tannates. Do not prescribe mercurous chloride (calomel) in a mixture that contains chlorides. Do not add water to preparations of resins, oleoresins, or to extracts or tinctures containing resins. Do not add water to official spirits. Do not add water to the tincture of strophanthus. DOSAGE The average dose of a drug or preparation for an adult is given in the United States Pharmacopoeia. This dose must be reduced for children, but must be increased in many instances to obtain desired results. Also, medical opinion is often at variance as to an average dose of a given drug, due, perhaps, to varying inertness or activity of a drug or preparation (and but few are really standardized), or due to some unusual personal experience. There is no one absolute dose of any drug; the dose is enough to do the work required; and, on the other hand, the symptoms of over-action or of cumulative effect should be quickly recognized. The average dose is modified by: (1) the age; (2) the weight; (3) abnormal physiology; (4) the pathologic condition present; (5) the frequency with which the dose is to be repeated. 1. Age. This is, of course, of importance in administering drugs to children, and the following is a simple rule for computing the dose: At 20 years the adult dose. At 10 years, half the age, half the dose. At 5 years, one-quarter the age, one-quarter the dose. If the child's age is between the ages given in the table, a little more or a little less, as the case may be, than the dose called for at the age in the table nearest the child's age will be the average dose. The table is simple and easy to remember, and represents the proper dosage for children, except when strong narcotic drugs are prescribed. The child's brain and central nervous system are relatively larger to its weight than in the adult, hence drugs that act strongly on the brain should be given in smaller doses than the above table would indicate. In old age the adult dose of strong drugs should often be reduced, as in the senile the physiologic processes are impaired, even if actual degeneration is not present, and hence a drug may cause excessive action. 2. Weight. The weight of a patient is really the scientific factor in determining the proper dose, hence the dose for an underweight child of five years should be less than that called for by the table, while an over-weight child of three years should often receive a dose for the five year period. A six-foot man requires a larger dose than a five-foot woman, if physiologic action of the drug is desired. However, over-weight due to fat and not size does not call for an increase in the dose, as fat does not represent an increase in the quantity of the blood, and it is the bulk of the blood (normally about one-thirteenth of the body weight) that really determines the dose. For ready reference it may be noted that: A normal baby five months old weighs about 15 pounds; at the end of a year, about 20 pounds; at the end of two years about 30 pounds; and from then on he should gain from four to six pounds a year, until at fifteen the child should weigh not far from 100 pounds. Up to this period the boy and the girl weigh about the same. Careful investigation of many thousands of accepted insurance risks-in other words, of well individuals-has developed the following average weights for age and height: A man's clothing weighs about seven pounds, his shoes with heels increase his height about one inch. Women on the average weigh about five to ten pounds less than men. An individual may weigh from twenty-five pounds below the average to twenty-five pounds above the average, and still be within the range of normal. Far below the average weight is more significant of latent disease under the age of thirty than above that age. Far above the average weight is more objectionable over the age of thirty than under that age. A family tendency to under- or over-weight is of importance in determining whether, or not, the individual is really abnormal and hence more likely to acquire disease or to become disabled. 3. Abnormal physiology.-Any abnormal physiologic process may modify the dose of a drug, such as any stomach or intestinal disturbance that delays absorption; a liver disturbance that impairs its antidotal action; or any impairment of the excretory organs, especially of the kidneys. 4. The pathologic condition present.-If any of the disturbances of physiology just suggested are caused by real pathologic conditions, the dose of a drug may need to be much less than the average, else poisoning may be caused. This is especially true in liver and kidney disease. On the other hand a pathologic condition may be present that requires a much larger dose than the average to overcome that condition and cause the effect desired, such as an hypnotic action in cerebral excitement; for a narcotic effect in colic; or for the effect of digitalis on the heart in broken compensation or in auricular fibrillation. 5. The frequency of the dose.-If a drug is slowly excreted, one dose of it a day may be of good size, while three or more doses a day must be smaller. A drug that is rapidly excreted may be given in full doses repeated as rapidly as it is known to be excreted. Alcohol, ammonia, and camphor act rapidly and are more or less rapidly excreted or rapidly destroyed in the body. Caffeine chloral, iodides, morphine, salicylates, strophanthus, and strychnine act rapidly, but are more or less slowly excreted or slowly destroyed in the body. Arsenic, atropine, bromides, digitalis, mercury, quinine, and most synthetic antipyretic and hypnotic drugs act slowly, and are excreted slowly. On account of the slow excretion, and because only fractional parts of one or more doses are excreted in twenty-four hours, certain drugs when daily repeated, tend to accumulate in the system. Such drugs are arsenic, atropine, bromides, digitalis and mercury. CONDITIONS MODIFYING THE ACTION OF A DRUG 1. Condition of the stomach, if a drug is given by the mouth. 2. Intolerance: idiosyncrasy. 3. Tolerance. 1. The condition of the stomach really means the varying rate of absorption. Theoretically, a drug received into an empty stomach should be more quickly absorbed than when the stomach contains food. This is generally true, as the drug is more rapidly passed into the intestine, where most absorption occurs. The stomach probably absorbs but few drugs, unless they long remain in the stomach, except alcohol and alcoholic preparations, probably morphine, and most alkaloidal salts. When for any reason there is inertia of the stomach, from shock or dilatation, or from sluggish circulation, a drug may remain for a long time in the stomach unabsorbed. Conse |