This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
Alimentary canal. - Excessive tea-drinking may cause indigestion, but this is probably induced by the tannic acid in the tea, and not by the caffeine. The teeth of tea tasters are very liable to decay. Coffee is, with some persons, slightly laxative; it is not known to what ingredient this is due.
Circulation. - Caffeine is freely absorbed. It produces no change in the blood. Moderate doses increase the force of the cardiac contraction and the duration of the systole, the diastolic period being shortened; as a consequence of this the blood-pressure rises. The pulse is usually slowed. Toxic doses paralyze the heart. These effects are largely due to the direct action of caffeine on the cardiac muscle, but also to an action on the inhibitory centres.
Vessels. - Caffeine causes first a constriction and then a dilatation of the arterioles of the body, and this is due principally to its action on the muscular coat and slightly to its action on vasomotor centres.
Respiration. - In animals the rate of breathing is increased by caffeine. Medicinal doses are said to excite and toxic doses to depress it.
Nervous system. - It is well known that tea and coffee stimulate the cerebrum. This is due to the caffeine in them. The patient becomes wakeful, the mental activity and capability for work are increased, the reasoning powers being quite as much affected as the imagination. In this respect the cerebral stimulation of caffeine differs from that of opium, and also in that the excitation is not inco-ordinate, nor is it soon replaced by sleep. Very excessive tea-drinking causes trembling of all the muscles of the body, and makes the patient extremely "nervous."
In man the spinal cord and muscles are very little affected by caffeine, but in some frogs the spinal cord is decidedly stimulated, and convulsions occur; in other species the muscles are thrown into a state of rigidity, which is clearly due to an action on the muscles themselves, for it follows the application of caffeine to an isolated muscle. Sometimes the muscle curve is altered in character. It is believed that in man the powers of muscular endurance are increased by caffeine. Motor and sensory nerves are uninfluenced in all animals.
Kidneys. - As caffeine first causes a contraction of arterioles there is decrease in the urinary flow; but soon the renal vessels dilate, the renal cells are stimulated and the flow of urine is increased. Thus caffeine is a good local diuretic.
Metabolism. - Many elaborate experiments have been made upon the action of caffeine on tissue waste; they are all of them inconclusive, probably because it has no effect. It increases the excretion of xanthin in the urine; probably this xanthin is derived directly from it, and the increased urea said to be excreted may also proceed directly from the caffeine. Toxic doses may cause a slight rise of temperature.
Heart. - Caffeine has been most used in heart disease. It is given when, as in aortic or mitral obstruction, a purely stimulant effect is desired; large doses, 3 or 8 gr. .20 to .50 gm. a day of the citrated, or 2 to 6 dr., 8. to 24. gm., of the effervescent citrated, are often easily borne, and may be combined with strychnine. These preparations are useful, when combined with antipyrin or acetanilid derivatives, to counteract their depressing influence upon the heart. Caffeine will not replace digitalis, for it does not slow the heart nor make it regular, and it shortens the diastole. It is, on account of its diuretic action, especially valuable in cardiac cases in which there is dropsy. Tea and coffee are, in some persons, liable to produce irregularity of the heart.
Kidney. - Small doses of caffeine are powerfully diuretic, and are therefore used in heart disease, ascites, pleuritic effusion, etc. As the drug acts directly on the kidney, it should be given cautiously in renal disease. Many patients so soon become used to it that at the end of a week it has lost its power of producing diuresis.
Nervous system. - Occasionally it cures migraine, but it is not so useful as antipyrin or exalgin.
It may be rendered sufficiently soluble for subcutaneous administration by mixing it with a solution of sodium salicylate.