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.
I have known iodide of potassium relieve many asthmatic patients, and Horace Green (1860) found it to be the main ingredient in a secret and successful remedy for asthma. Trousseau and Jaccoud speak of its value, and M. See records valuable observations upon twenty-four cases watched for a long time. Four of these were children, four old people, the others adults; the daily dose varied from 22 to 45 gr., it being reduced as improvement progressed; if given some hours before the usual attack, this was often prevented; if given during it, respiration was rendered free in one to two hours. Chronic asthma with emphysema was also benefited by the remedy; inhalations of the iodide of ethyl, six to ten drops several times daily, and the occasional use of opium or chloral in these latter cases, were with advantage conjoined with the treatment (Medical Record, 1878). Dr. Hyde Salter has observed benefit from iodide of potassium in full doses - 15 to 30 gr. - every two to four hours, in very diverse cases of asthma. I think that such attacks as are connected with Catarrh and are relieved by free secretion, and in which the nerve-symptoms are reflex, rather than primary, show the best results from this remedy. I have known it efficacious in asthma connected with amenorrhoea and uterine congestion, and also in the asthma of rheumatic and gouty subjects. In an interesting case in a very rheumatic patient, the asthmatic attack was relieved by 4-gr. doses of iodide; but severe pain in the region of the kidney followed, with secretion of scanty, acid urine; this occurred more than once, and was only relieved by free excretion of alkaline urine under appropriate remedies (British Medical Journal, January, 1875). In this case the drug was supposed to cause renal congestion by increasing the absorption of waste nitrogenous material, and consequently the amount to be eliminated. I have known iodine itself produce renal congestion in some individuals.
Dr. C. J. B. Williams has seen a very large number of asthmatic cases relieved by iodide and by carbonate of potash with stramonium (Medical Times, i., 1872), but most of M. See's cases were relieved by the iodide alone. Dr. Reed recommends the liquor iodinii in "dry asthma" of constitutional character and without obvious exciting cause (Medical Record, 1879).
I believe that the drug acts directly on the mucous membrane, relieving its congested state by promoting a thin, fluid secretion; but, independently of any theory, it will be found worthy of trial in any rebellious case.