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.
From the time of Girdlestone (1806), Willan and Bate-man, Biett and Cazenave, arsenic has held the first place in the treatment of this malady. Modern dermatologists agree upon this point, but some, as the late Mr. Startin and Mr. Hunt, rely upon this drug much more positively than others. The preference of Hebra for local over any constitutional treatment is well known, but even he recognizes "a decided curative action of arsenic in this form of disease." Dr. Tilbury Fox inclines to restrict its use to the more typical cases, especially those of chronic character and accompanied with nerve-debility; on the other hand, many cases will be found to occur in persons otherwise strong, and in such cases after preliminary purgative treatment I have found the remedy useful. "Before undertaking to deal with psoriasis," remarks Gaskoin, "it is necessary to know all about arsenic" ("Treatise on Psoriasis"). Its success, however, is, as Stille remarks, by no means uniform, and any want of due attention to the excretions, to the presence of gouty or other constitutional tendencies, or to the proper regulation of dose, will readily prevent a satisfactory result. It can by no means be considered a specific, but as a valuable agent only under certain conditions; neither can it be accurately stated that "the more chronic the malady the more suited it is for this remedy," for after it has lasted for eight or ten years I have seldom found it amenable, and Devergie has recorded a similar experience ("Maladies de la Peau").
Mr. Malcolm Morris notes that sometimes arsenic not only does no good in psoriasis, but harm, in rendering the patches more hyperaemic and irritable; he finds it impossible, however, to diagnose the cases in which this may occur (Practitioner, 1880).
1 The reason for doubting the value of arsenic in herpes is that it is a disorder of definite course, which must develop, but the neuralgic pain, if severe, is markedly lessened by arsenic (v. p. 67).
In judging of its true power, we must allow for the natural improvement of the malady in certain circumstances, e.g., on the cessation of lactation, at changes of climate or of season, etc., also for the effect of external treatment by tar or bathing carried on at the same time. But after these allowances there remain, no doubt, many cases which show improvement distinctly from arsenic; the best illustrations are seen in children, and then in older persons in whom the attack is comparatively recent yet not in an acute stage; chronic cases that have been left untreated often do well, but previous irregular trials diminish the chances of recovery. In any case, if cure be effected, freedom from relapse cannot be guaranteed (Wilson); Hunt has shown how important it is to secure a due action of the absorbents, and also that one preparation may succeed when another has failed; for instance, to one of two girls similarly affected, he gave the potash, and to the other the soda solution; for a time both did well, and then both ceased to improve, but later on, when he exchanged their medicines, they progressed to cure (Journal of Cutaneous Medicine).