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.
There is much difference in the susceptibility of persons to the action of iodine, and we can explain this in no better way than as "idiosyncrasy." Speaking generally, it may be said that pale, thin, languid patients often bear it better than the stout and plethoric, who have a tendency to active head-congestion or stasis of circulation. Such subjects, if rheumatic, are often intolerant of even small doses; so are the sufferers from goitre or exophthalmos, or nervous palpitation accompanied with irregular flushing and impaired vaso-motor power.
Climate and soil seem to have some influence, for Coindet's patients in Geneva were much more susceptible than those of Ricord in Paris (Medico-Chirurgical Review, ii., 18G0). Dr. Lisson concluded that people with fair skin were more susceptible of the action of this, and all other drugs affecting the skin, than dark subjects, and this may prove some guide. He himself was able, by commencing with small doses, to induce a state of toleration, so that he could take 100 gr. of iodide of potassium without marked effect (Lancet, i., 1860). On the other hand, 3-gr. doses have caused severe inflammatory attacks (Lancet, ii., 1873, p. 119), and even a less quantity may excite distressing coryza. Bad effects may, however, be often avoided by following Lisson's plan, and inducing tolerance, e.g., I gave a patient with syphilis iodide of potassium in 3-gr. doses thrice daily, but he always suffered from coryza and headache after two or three days; then it was reduced to 1 gr., and the same results followed; then he took 1/2 gr. thrice daily and had no ill-effects, and the dose was gradually increased, and in a few weeks he was able to take 15 gr. three times daily without any bad symptoms. In one (exceptional) case, iodism setting in after the fourth dose of 3 gr. of iodide of potassium, affected the larynx so intensely as to require tracheotomy (Lancet, ii., 1875); and, as already mentioned, Dr. S. Mackenzie traced fatal purpura in an infant to 2 1/2 gr.; but the question may be raised whether the salt was quite pure in such cases. It has also been mentioned that evil results have been attributed to an admixture with iodate of potash, and in such exceptional cases an analysis should be made with reference to this point. Acute iodism is generally proportioned to the largeness of the dose; but the chronic condition is more readily induced by continued small doses. The iodides of sodium or ammonium are often (not always) better borne than the potassium salt.