This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Iodine has a pungent taste, and in small doses causes heat and stimulation of stomach, with some increase of appetite. A dose of more than 1 gr. usually causes sickness, and 5 gr. give rise to salivation, pain in the abdomen, and diarrhoea; large doses may cause glossitis, local inflammation, and ulceration. Vomiting, burning pain, spasm, choking sensation, and impairment of the special senses, were symptoms noted by Mr. Bainbridge after the taking of 1 oz. tinct. iodine, British Pharmacopoeia; oil was given, and recovery occurred gradually (Lancet, ii., 1875).
It is remarkable that iodoform, though containing so large a percentage of iodine, does not usually irritate the gastric mucous membrane, unless in toxic doses. The alkaline iodides readily disorder the stomach in many persons, and though at first they may increase appetite, they afterward impair it. Small quantities are apt to constipate, but, if continued they produce diarrhoea, with liquid, slimy stools. At times, gastric irritation and catarrh are the only marked symptoms of iodism (Ril-liet). Leroy (Brussels) has adduced evidence to show that when gastric pain is caused by iodide of potassium, it is really due to adulteration with iodate (Medico - Chirurgical Review, ii., 1857). Mialhe endorses this, and Melsens considers such adulteration dangerous; five dogs were poisoned by it ("Memoire," Brussels, 1865).
Rabuteau points out that either of the salts separately is unacted upon by weak hydrochloric acid, while their mixture is quickly decomposed by it, with liberation of free iodine; also, if fresh gastric juice be mixed with starch in test-tubes containing the one iodide, and the other iodate, no blue reaction occurs till the contents of the two tubes are mixed, implying that free iodine is the irritant agent in the impure salt, and that a pure alkaline iodide is non-irritant. Practically, however, I am satisfied that as pure an iodide as is obtainable will produce gastric irritation in some subjects. In the case of all iodine compounds, such irritation may be avoided or lessened by giving them freely diluted, and shortly after food.