Too little attention has been bestowed upon the mutual relations of food and medicines. So little is really definitely known of the intricate chemistry of digestion and assimilation that it is difficult to formulate rules for the right time of giving every drug in relation to fulness or emptiness of the stomach. The reaction of the stomach contents varies from alkaline to neutral and acid, and these several reactions will decompose medicines in various ways. Besides this, the reactions themselves are dependent upon a large number of organic acids, salts, and other substances which may wholly alter the composition of a medicine at one time, and not be present to affect it at another.

A drug given after a full meal may be decomposed by the strong hydrochloric acid of active digestion, which is unaltered in an empty stomach.

Conversely, remedies which are not themselves influenced by the gastric and pancreatic juices may affect these secretions as synergists or the reverse. Chittenden says: "Take, for example, the influence of such substances as urethan, paraldehyde, and thallin sulphate on the proteolytic action of pepsin-hydrochloric acid, and we find that small quantities (0.1 to 0.3 per cent) tend to increase the rate of proteolysis, while larger amounts, say one per cent, decidedly check proteolysis. Similarly, among inorganic compounds, arsenious oxide, arsenic oxide, boric acid, and potassium bromide in small amounts increase the proteolytic power of pepsin in hydrochloric-acid solution, while larger quantities check the action of the ferment in proportion to the amounts added. Again, with the enzyme trypsin, similar results with such salts as potassium cyanide, sodium tetraborate, potassium bromide and iodide may be quoted as showing not only the sensitiveness of the ferment towards foreign substances, but likewise its peculiar behaviour - viz., stimulation in the presence of larger quantities.

Furthermore, we have found that even gases, as carbonic-acid and hydrogen-sulphide, exert a marked retarding influence on the proteid-digesting power of trypsin." These gases, being a product of intestinal malfermentation, may thus interfere with digestion.

Hydronaphthol and bismuth salicylate retard gastric digestion. Sodium bicarbonate and other alkalies check pyrosis in chronic gastritis, which is caused by diminished hydrochloric-acid secretion, but increase pyrosis subsequently, because of the greater alkalinity produced, which favours the growth of lactic-acid organisms. According to Leffmann and Beam, beta-naphthol stops the action of diastaste, but not that of the amylolytic ferment of pancreatin, hence it is useful as an intestinal antiseptic without wholly stopping digestion. It, however, retards proteid digestion. They also state that salicylic acid and saccharin both prevent the amylolytic action of diastaste and of pancreatin, but do not retard proteid digestion.

These few examples are sufficient to illustrate the very diverse influence of some of the common drugs.

The following rules are subject to many exceptions, but they will serve as a general guide: