II. In considering drugs in reference to the second division, we find them grouped according to the effect they have on the human organism in disease. This mode of classification must be grasped broadly, not by rule. It is impossible to assign fixed and definite places to all medicines from this standpoint, or to draw sharp lines of division. Remembering the complex nature of plants, and that one drug may contain more than one active alkaloid, and a number of the less important substances just named, it is evident that one medicine may have several actions; and practically it is found that their effects are as scales of varying gradation, and run insensibly into one another. The same drug that in small doses acts as a beneficent agent may in large ones be an overwhelming poison; one given to quiet the nervous system may also constipate; another given for purgation may seriously reduce the strength of the heart; another given to strengthen the heart may also increase the flow of urine, so that classification may well be sometimes a little puzzling.
It will dispel bewilderment to remember that the most important and prominent characteristic of a drug is usually taken as its representative quality, the others being for the time ignored.
Individual peculiarities have much to do in modifying the physiological actions of drugs. The more highly strung nervous organizations respond more quickly, as a rule, to the actions of drugs than do those of coarser fibre, and more quickly show evidence of over-dosing and mild poisoning. Among these temperaments are found many examples of what is called "idiosyncrasy" - that is, an increased susceptibility to the effects of a drug which entirely forbids its use, and for which no reason can be discovered.
Custom makes a decided difference, and the action of a medicine is more energetic with one unused to it. With frequent repetition comes "toleration," when the system accommodates itself to the drug, and larger doses can be taken with relatively less effect. Beyond this point comes "habit," when the system not only tolerates but craves the drug in ever increasing quantities, and with the result of a progressive degradation of the will power, as is most strikingly shown in the ascendency of alcohol and opium over the individual.
"Accumulation" and "cumulative action" of drugs are expressions often met and are self-explanatory up to a certain point. While it might be difficult to explain the exact processes in the body tissues by which drugs are stored up or accumulate in them, it is, fortunately, sufficient for practical purposes to know that a number of drugs do become apparently fixed in the tissues, and that many others accumulate by being given more rapidly than they can be excreted.
Age is an important factor to consider. Strong drugs, and especially those that act on the brain, are given with much care to children and to the aged. The condition of the stomach is another point to consider. Medicines act more rapidly on an empty stomach, and any irritating properties they may have are then more marked. Given with or soon after food the action is more gentle and slow. The kinds of food taken are to be thought of, and any which might neutralize the medicine should be put off for a safe interval. In giving medicines to produce sleep, all conditions favorable to sleep must first be secured. The good effect of many a hypnotic is lost through the failure to provide darkness, warmth, and quiet before giving it.
In general the effects of medicines are classed as primary and secondary, immediate or remote. By the primary or immediate action is meant the first definite result of the drug, and subsequent changes which are brought about by this first result are termed the secondary or remote effects, viz., if a diuretic is administered to a dropsical patient a copious flow of urine soon occurs as the primary effect. As a result of increased urination fluid is largely abstracted from the body tissues, and the consequent diminution of the dropsy is the secondary effect. The classes of medicine according to their physiological actions are arranged as follows: