Carbolic acid is a powerful antizymotic, rapidly destroying organized ferments, both animal and vegetable. Consequently it destroys those of septic diseases, hence it is antiseptic. It thus prevents the formation of the products of the decompositions which are set up by these organisms. For this reason it is disinfectant, and as the products of decomposition are generally foul-smelling, it is deodorant. It does not act so readily on unorganized ferments (enzymes), such as pepsin and ptyalin, but in large doses it likewise destroys their activity. Carbolic acid is not so powerful an antizymotic as corrosive mercuric chloride (see p. 214); for Evans found that anthrax spores were not killed in twenty-four hours by a solution of 1 in 100, but were killed by a solution of 1 in 20 acting for twenty-four hours, but not when it acted for only four hours. The bacilli of anthrax were killed by a solution of 1 in 100 acting for five minutes, 1 in 150 acting for a quarter of an hour, 1 in 175 acting for half an hour, but were unaffected by a solution of 1 in 150 acting for one minute, 1 in 175 acting for a quarter of an hour, 1 in 300 acting for an hour. Strengths of 1 in 40 and 1 in 20 are commonly employed in surgery. The solution in oil has no antiseptic properties. The power of carbolic acid to destroy low organisms makes it an efficient parasiticide against certain vegetable parasites infesting the skin.

When applied to the skin in weak or moderately strong solutions, it produces local anaesthesia with a feeling of numbness which lasts some hours. If concentrated, it acts as an irritant and caustic, causing a burning pain, and in a few minutes a white spot appears, which becomes red when the acid is removed. If the application is prolonged, a white eschar or slough results. There is no vesication.


Gastro-intestinal tract. - If concentrated, carbolic acid produces the same effect on the mouth as on the skin, and is a powerful gastro-intestinal irritant (see Toxicology). In the stomach it is converted into a sulphocarbolate, and unless poisonous doses be given, it is so diluted by the gastric contents that it loses its antizymotic power.


It is not known in what form carbolic acid circulates, probably as an alkaline carbolate.


Medicinal doses have no effect. Large doses paralyze the vaso-motor centre in the medulla, and the blood-pressure falls. It is not until very large doses have been given that the heart is affected, and then its activity is depressed.


Small doses have no influence on respiration, but large ones accelerate it, probably from stimulation of the vagi. Ultimately respiration is paralyzed, and death results.


This is unaffected by small doses of carbolic acid, but large doses cause it to fall, because they diminish the production of heat and increase its dissipation.

Nervous System

Carbolic acid is a cerebral depressant in large doses, for coma is produced by them; they first stimulate the anterior cornua, producing convulsions, but subsequently depress them, causing paralysis.


Much interest attaches to this, for even after moderate doses of carbolic acid, or absorption from surgical dressings, the urine may become dark. This is not due to blood, as was once thought, for Stevenson has shown that there is no increase of iron in the urine. Carbolic acid is partially oxidized to pyrocatechin and hydroquinone, which combine in the body with sulphuric and glycuronic acids and are excreted in the urine as double (ethereal) sulphates and phenol, pyrocatechin and hydroquinone glycuronates. Pyrocatechin and hydroquinone are unstable bodies, and their oxidation products are doubtless the cause of the dark urine; pyrocatechin can only exist in alkaline urine, so that it cannot be the sole cause of the dark color. The presence in the urine of these results of carbolic acid is recognized by distilling them over from it. The distillate gives a blue color with neutral ferric chloride, and a white crystalline precipitate of tribromophenol with bromine water, showing the presence of sulphocarbolic acid. The inorganic sulphates are usually absent. This is determined by the use of the barium chloride test which does not precipitate the combined sulphates (sulphocarbolates). (Sonnenberg's test). Some carbolic acid escapes in the other excretions; some is burned up in the body. When very large doses are given carbolic acid itself may appear in the urine.