When the action of a drug is spoken of, the physiological action is usually understood.

The primary action is that due to the unaltered drug; e.g., the emetic action of zinc sulphate.

The secondary action is that due to compounds formed from the drug whilst it is in the body; e.g., the antiseptic effect on the urine of Uva Ursi taken by the mouth is probably due to the fact that arbutin, the active principle of Uva Ursi, is in its passage through the kidney decomposed into a glucoside and hydroquinone, and the latter is a powerful antiseptic.

The direct or local action of a drug is that produced on any organ with which it comes in contact e.g., the cantharidin in cantharides, in being excreted through the kidney, causes inflammation of it.

The indirect or remote action is a secondary effect, the result of the direct effect e.g., curare paralyzes the respiratory muscles, consequently the blood becomes venous, and therefore convulsions take place. In this case the venosity of the blood and the convulsions are each of them indirect actions of curare.

It is clear that among drugs acting on the same parts, the total effect will depend very much upon which part is first affected. For example, atropine and curare will paralyze motor nerves, but atropine first affects the terminations of the vagus, and only late in its action the motor nerves of the voluntary and respiratory muscles; hence paralysis and asphyxia are late symptoms, and a rapid pulse is an early symptom. Curare, however, early affects the nerve-endings of the voluntary and respiratory muscles, and the heart towards the end; therefore asphyxia and paralysis occur early, and a rapid pulse is a late symptom.

Relation Between Chemical Constitution And Physiological Action

There is no doubt that the physiological action of a drug often depends upon its chemical constitution. Naturally, substances which are broken up in the body in such a way as to lead to the liberation of a common element or group will have a similar action. Probably, also, the action of a drug depends upon electrolytic dissociation of its solutions.

Instances in which chemical constitution influences action are the similarity of effects of nitrites, the fact that all chlorides, bromides, and iodides of ethane and methane are anaesthetic, the similarity of action of the iodides of many metals and the similarity of action of bromides of many metals.

Substitution of one radical for another in organic compounds often strikingly modifies their action; for example, if strychnine, brucine and thebaine are converted into meihyl-strychnine, methyl-brucine and methyl-thebaine, the convulsive action of each of the first three substances is replaced by a paralyzing action. The effect of substitution may be also well seen in the various derivatives of atropine and cocaine, and in the relation of aconitine to benz-aconine and aconine. Another very interesting case in point is that methyl-glucoside is sweet, ethyl-glucoside is somewhat sweet, phenyl-glucoside is bitter, and benzyl-glucoside is intensely bitter.

Sometimes the position of the radicals in the molecule is of great physiological importance; thus resorcin (metadihydroxy-benzene) is very sweet, while pyrocatechin (orthodihydro-benzene) is bitter.

Sometimes the atomic weight appears to influence the intensity of action, for the relative toxicity of various alcohols is as follows: Methyl-alcohol, 0.8; ethyl-alcohol, 1.0; propyl-alcohol, 2.0; butyl-alcohol, 3.0; amyl-alcohol, 4.0. The difficulty of the whole subject is, however, so great that it is impossible at present to lay down laws sufficiently general to be of any use to the beginner. It must be remembered that dissimilarity of action is often more apparent than real, for it may be due to varying solubility, digestibility, rate of absorption, rate of elimination, or rate of osmosis, also to the organ which happens to be first affected, and the degree to which the drug can dissolve the constituents of tissues.

Drugs may be classified according to the parts on which they act, and before describing each individual drug, a classification on this principle will be given.