Drugs may act as such:

1. Independently of the human body, as antiseptics on microorganisms in disinfection.

2. In or about the human body, but not on its structures, as in the destruction of a tape-worm, skin parasites, etc., or as in the neutralization of a hyperacid gastric juice by an alkali.

3. On the structures of the human body. Drugs may act on the tissues - (a) Through their physical or mechanical properties, as when cold cream is applied to a chapped face to soften the epithelium and prevent its drying; or when bismuth subnitrate, given for diarrhea, coats the mucous membrane of the bowel and soothes and protects it. Or they may act (b) by their chemic affinity for one or other constituent of protoplasm, so that either the functional power of the cell or the actual cell structure is changed. Some of these are general in their action, affecting practically all forms of protoplasm (though not all forms to a like degree), and when the action of these drugs is powerful, they are known as general protoplasm poisons. Such are alcohol, chloral hydrate, and quinine. Other drugs are selective, exerting their influence only on special groups of cells and having no effect upon the vast majority of body structures. This is presumably owing to a chemic affinity for some component of the cell. Such drugs are strychnine, which has a selective affinity for certain portions of the central nervous system, and pilocarpine, which has an affinity for secretory nerve-endings.

The effect of drugs on cells is to stimulate them, to depress them, or to change and destroy them. Stimulation is an effect on cells by which their power or their readiness to functionate is increased. Depression is an effect on cells by which their power or readiness to functionate is lessened. Paralysis is the cessation of the power to functionate.

Irritation implies an anatomic rather than a functional effect, tending toward the harmful. It has to do with actual changes in the cell structure. In its mild degrees irritation may have the effect of stimulation; in stronger forms irritation may overwhelm the cells and have the effect of depression; while excessive or continued irritation induces inflammation and even actual death of the cells involved. As an example, take cantharides, an irritant to the kidney cells; from small doses the cells are made to functionate more actively, and increased urination takes place, but from toxic amounts the irritation results in inflammation, so that nephritis sets in, with destruction of cells, impairment of function, and, perhaps, suppression of the urine.

By exhaustion from overwork, continued stimulation may result in depression or even complete cessation of the work of the cells, but this is a functional inactivity from fatigue, and a period of rest and nutrition will usually restore the cells' power.

Often a drug will be found to stimulate one structure and depress another, as atropine, which stimulates the vagus center and depresses the vagus endings; or pilocarpine, which stimulates the nerve-endings in the sweat-glands and tends to depress heart muscle.