This section is from the "A Handbook of Useful Drugs" book, by State Medical Examining and Licensing Boards.
Bismuth subnitrate is a basic bismuth nitrate of somewhat varying chemical composition.
Properties : Bismuth subnitrate occurs as a heavy, white, odorless and almost tasteless powder. It is practically insoluble in water and in alcohol and is little affected by weak acid solutions. Hydrochloric acid of the usual strength of the gastric juice decomposes only a small amount in the course of several hours. Hydrochloric or nitric acid, if not too dilute, decomposes it, producing the chlorid or nitrate, which enters into solution ; but when this solution is diluted with water the insoluble basic chlorid (oxychlorid or subchlorid) or basic nitrate (subnitrate) is precipitated.
Basic bismuth nitrate is rendered more basic by the action of alkali until finally it is converted into bismuth oxid, a nitrate of the alkali metal being formed. Under some circumstances the nitrate radical may be reduced to nitrite, especially by the putrefactive bacteria of the large intestine.
Incompatibilities: It is incompatible with acids, tannins, sulphids and sulphur. With soluble carbonates and bicarbonates in the presence of water there is a liberation of carbon dioxid, a formation of insoluble bismuth compound, and the nitrate of the alkali metal. With iodids a double decomposition has been noted with the formation of the red basic iodid of bismuth and the nitrate of the metal whose iodid was used.
Action and Uses: Soluble bismuth compounds, as a rule, become converted in the presence of water into insoluble basic compounds. Most of the preparations used in medicine are already in the basic form. (As the action of bismuth preparations depends on the action of an insoluble powder, there is no object in prescribing a soluble salt.) The salt most frequently used is bismuth subnitrate; but since it sometimes produces poisonous effects, the subcarbonate is the preferable salt. All the compounds of bismuth used in medicine produce essentially the same effects.
Bismuth subnitrate is not appreciably affected by the gastric juice and does not materially lessen its acidity. When given in considerable doses it coats the mucous membrane and acts as a mechanical protective. It thus prevents the action of the digestive secretions and of irritating foods or other substances on the mucous membrane. The same is true of its effect on the mucous membrane of the intestine. It is not absorbed in the stomach. It undergoes chemical changes in the intestine and is probably absorbed there to some extent though seldom in sufficient quantities to produce symptoms of poisoning. It is excreted almost entirely by the cecum and other parts of the large intestine. It is turned black in the large intestine, probably from contact with sulphids. It seems to exert an astringent effect on the gastrointestinal mucous membrane. When applied to the skin it acts mechanically, but on wounds and ulcers, as on mucous membranes, it acts as a protective, astringent and antiseptic. It is absorbed from wounds to a larger extent than from mucous membranes. A number of cases of poisoning have been so caused.
Bismuth subnitrate and other insoluble salts of bismuth are used in irritation of the stomach and intestines for their protective and astringent powers. They are useful to allay vomiting from gastric irritation. In the same manner they serve to check diarrhea, especially that arising from the ingestion of irritating foods. They are useful in hyperacidity and ulcer of the stomach by coating and protecting the mucous membrane. They are also employed in catarrh of the stomach and intestines. They may be given for the same purpose in ulcerative enteritis. Externally the subnitrate and subcarbonate are used as protective and antiseptic applications in skin diseases and as applications to ulcers or suppurating wounds and to promote the healing of old sinuses and fistulous tracts. In the latter case the bismuth is used in the form of a paste, combined with petrolatum and wax. A number of cases of poisoning have been reported due to the absorption of the bismuth; hence care must be exercised in its use and on the appearance of toxic symptoms, such as a blue line on the gums, headache, nausea and stomatitis, the bismuth should be removed from the fistula by the injection of warm olive oil.
Dosage: 1 gm. or 15 grains. For the treatment of ulcer much larger doses are used. Externally it is used freely as a dusting powder or in ointment. Very large quantities are used for the purpose of outlining the stomach and intestines by roentgenography and by the fluoroscope, but the occasional occurrence of nitrite poisoning has led to a preference of the subcarbonate or oxychlorid for Roentgen-ray work. Poisoning by nitrites is indicated by vasomotor paralysis, tachycardia and asphyxia due to the formation of methemoglobin.