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..
Benzoic acid is a powerful antiseptic. The growth of many forms of bacteria is completely inhibited by a solution of 1 in 1000. In a concentrated form it is a stimulant and irritant when applied to the skin.
The chief fact about the internal action of benzoic acid that has been worked out is that when it is given by the mouth, hippuric acid appears in the urine. This happens by combination with a molecule of glycocoll, Hc7h5o2+C2H5 No2=C9H9No3 (hippuric acid)+H2O. The source of the glycocoll is not known. The conversion takes place in the kidneys, for after giving large doses of benzoic acid, it alone can be found in the blood, and if the renal arteries are tied, no hippuric acid is formed, but if only the ureters are tied it is formed. Also benzoic acid has been successfully converted into hippuric acid by passing blood, containing benzoic acid, but no glycocoll, slowly through the kidneys removed directly after death. Further researches show that the conversion is probably effected by the renal cells. Hippuric acid has been found in the urine of new-born children when benzoic acid has been given to the mother shortly before delivery. If hippuric acid is given by the mouth benzoic acid is found in the blood, but hippuric acid reappears in the urine. The hippuric acid in the urine renders alkaline urine acid, and it stimulates and disinfects the urinary mucous membrane. Occasionally succinic as well as hippuric acid appears in the urine.
Benzoic acid or some derivative of it is probably excreted in the bronchial secretion, for the bronchial mucous membrane is stimulated by the administration of benzoic acid, the mucus being increased in quantity and disinfected. The acid is therefore expectorant. The same effects are brought about if the vapor of benzoic acid is inhaled. It is said also to be excreted by the skin and salivary glands, and thereby to increase their activity. It is slightly diuretic. Medicinal doses do not produce any effect on the stomach, intestines, circulation, or nervous system.
Benzoic acid and its salts are antipyretic, and it is stated that they are even more powerful than salicylic acid. How they produce a fall of temperature is not known. Metabolism is believed to be generally increased. Sodium and ammonium benzoate increase both the quantity and the solids of the bile.
Lint soaked in the compound tincture is a favorite dressing for wounds and sores of all sorts. Its chief advantage is the antiseptic power it possesses. Its stimulating effect is also valuable. Benzoinated lard is a common basis for ointments when it is wished that the active ingredient should be absorbed, for the lard melts on the body, especially if covered with a bandage; the benzoin prevents the decomposition of the lard. If the benzoin irritates, which it is likely to do if near the eye, 3 parts of oil of cloves or 2 of oil of gaultheria to 480 parts of lard makes a non-irritant basis which keeps indefinitely.
Lungs. - Benzoin, benzoic acid and its compounds are very commonly employed as stimulating, disinfecting expectorants in cases of bronchitis or phthisis in which the expectoration is foul and scanty. The vapor from a mixture of a pint, 500 c.c. of water at 140o F., 60° C and a fluid drachm, 4. c.c. of compound tincture of benzoin is often inhaled for bronchitis and laryngitis.
Urinary organs. - Benzoic acid is a most valuable drug for acidifying the alkaline decomposing urine which is formed in pyelitis and cystitis, and for stimulating and disinfecting the . urinary tract in the same conditions. Ammonium benzoate is so much more soluble than benzoic acid that it is to be preferred to it. Spirit of chloroform covers the taste. It may with advantage be combined with the urinary sedatives, as tincture of hyoscyamus. The conversion to hippuric acid has been said not to take place when the kidney is diseased.
Benzoic acid has been used in Germany instead of salicylic acid for rheumatic fever.