Iodide of potassium 64 grains, carbonate of potassium 4 grains, distilled water 1 fluid drachm, prepared lard 1 ounce. Dissolve the iodide of potassium and carbonate of potassium in the water, and mix thoroughly with the lard. B.P.

Iodide of potassium 12, hyposulphite of sodium 1, boiling water 6, benzoated lard, 81. U.S.P.

The ointment is apt to become discoloured by the liberation of free iodine when iodide of potassium and lard only are used. The carbonate of potassium, B.P., is added in order that it may combine with any iodine set free, and the hyposulphite, U.S.P., is also used to prevent this discoloration.

Action. - The action of iodide of potassium appears to depend partly on the iodine and partly on the potassium it contains. It differs from that of free iodine (p. 558) in being much less irritant. On this account it is of little use as a local stimulant but it can be given in much larger doses. It has been supposed that iodine is set free from iodides in the stomach; but probably this is not the case, at least to any great extent, unless the iodides are contaminated with iodates. Iodide of potassium and other alkaline iodides are readily absorbed. It is conveyed by the blood to the various tissues of the body. It has been supposed by Binz to be partially decomposed by some of them, with the evolution of free iodine both in the blood and in the tissues, and he attributes its most important actions to this decomposition. The iodine set free from the iodide is taken up by albuminous substances, and the entrance of the iodine molecule into their composition causes them to undergo more rapid metamorphosis. Gummatous deposits appear to be especially affected in this way.

Lead and mercury also appear to be set free by it, from their combinations with the tissues, and entering once more into the circulation are eliminated. Iodides are eliminated very rapidly by the kidneys, salivary glands, probably by all mucous membranes, and by the skin. During the process of elimination iodine is occasionally set free and causes local irritation. This is especially marked in the mucous membrane of the nose, and in the skin, but it may occur also in the conjunctivae, bronchi, and stomach. The irritation of the nasal mucous membrane thus produced gives rise to the symptoms generally known as iodism. They are exactly the same as those produced by prolonged exposure to the fumes of iodine. They consist of running at the nose, and frontal headache, which probably depends upon swelling of the mucous membrane lining the frontal sinuses. There is also frequently running of the eyes. Not unfrequently the bronchial mucous membrane becoming congested there is cough and pain in the chest. These symptoms are most readily produced by small doses of 2-5 grains, and they may usually be arrested either by discontinuing the medicine or increasing the dose. When the dose is raised to 10 grains the symptoms usually disappear, and I have only seen one case in which they persisted after the dose had been raised to 30 grains. In some persons the congestion is not confined to the nose, but extends to the back of the throat and to the larynx, so that serious symptoms of suffocation may follow the laryngeal congestion produced in them by iodide. As the iodine is eliminated in the tears, severe conjunctivitis may follow the application of calomel to the eyes of persons who are taking iodide at the same time. Affections of the skin usually occur with large doses of iodide. The most common form of eruption is acne, but tubercular eruptions are also met with. They appear to be caused by decomposition of the iodide with elimination of free iodine in the sweat and sebaceous matter. They are said to be lessened by the simultaneous use of arsenic, and to be prevented by perfect cleanliness and daily baths. Occasionally the iodide causes gastric irritation with diminished appetite. It is readily excreted by the salivary glands, and may give rise to salivation (p. 358). It sometimes gives rise not only to congestion of the bronchial mucous membrane and cough, but to haemoptysis, exudation into the pleural cavity, and even pneumonic consolidation.

In some persons it greatly depresses the genital functions.

During its excretion by the kidneys it acts as a diuretic, though not a very powerful one.

Uses. - Although iodide of potassium is probably absorbed in very small quantity by the unbroken skin, even when mixed with oil or fat, yet the iodide of potassium and soap liniment, especially when mixed with its own bulk of opium liniment, sometimes gives considerable relief when applied to inflamed and rheumatic joints by means of flannel or lint. When used with lanolin, it is said to be more readily absorbed and to give still greater relief in chronic joint disease. Iodide of potassium is chiefly used, however, internally in syphilis, rheumatism, scrofula, and chronic poisoning by lead or mercury. In the primary and secondary stages of syphilis, mercury is generally used either alone or in combination with iodine. In the tertiary stage, iodide of potassium is more generally given alone, although it is said by some to have but little effect unless mercury has been administered at some previous time. If this opinion be correct, the beneficial action of iodide of potassium may be due, in part at least, to its again liberating part of the mercury which has been in a state of more or less dormant combination with some of the tissues. The powerful action of iodide of potassium in removing syphilitic deposits is readily seen when these deposits are superficial, as nodes on the shin or on the sternum, or when they can be readily seen, like deposits in the larynx. Sometimes such deposits are unaffected by small doses, such as five grains of iodide, but disappear rapidly when the dose is increased to ten grains or more. From its beneficial action on visible deposits we may infer that it has a similar action on those which are deeply situated, and indeed sometimes we may observe enlargement and induration of the liver, probably dependent on a syphilitic condition, rapidly disappear under the use of the iodide. In chronic rheumatism, especially when the pain is worse at night, it is sometimes useful.

It apparently increases the activity of the lymphatic system, and is used in enlargement of glands connected with this system, e.g. enlarged thyroid, enlarged spleen, and the enlarged lymphatic glands which occur in scrofula, as well as in scrofulous conditions generally.

It is given wherever absorption is deficient and organs become hypertrophied, e.g. the breasts, testicles, prostate, uterus, ovaries, etc. In cancer and tubercle it is of little benefit; it is sometimes given, and possibly with benefit, in order to aid the absorption of pneumonic consolidation.

In bronchitis with much congestion and deficient secretion it is a useful expectorant, rendering the mucus more abundant and less tenacious, so that it is more readily expectorated.

As syphilitic skin-diseases often disappear under its use, it has been applied to other skin-diseases not dependent on syphilis, such as psoriasis, lepra, herpes, impetigo, lichen, prurigo, sycosis, acne, lupus, etc, especially in scrofulous patients.

In frogs it destroys sensibility and voluntary motion by acting on the spinal cord. It is useful in large doses to diminish the pain in cases of aneurism, and is also used in neuralgia, paralysis, convulsions, etc.

The relief which it affords to the pain of aneurism is very marked, but it must be given in large doses, e.g. thirty grains. The benefit which it affords may be partly due to weakening of the circulation, partly to diminished sensibility by the action of the drug on the nervous system, and partly to beneficial alterations in the morbid condition of the walls of the affected vessels, which are often syphilitic in character.

It is exceedingly useful, as already mentioned, in chronic metallic poisoning, e.g. by mercury or lead.

It is used in dropsies as a diuretic, and is also employed as an emmenagogue.