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.
Magnesia forms a rather insoluble salt with arsenious acid, and is ordered in the German Pharmacopoeia as part of the officinal "antidotum arsenici." 1
1 Seven parts of magnesia mixed with 120 parts of water are to be kept in one bottle, and 60 parts of liquor ferri persulphatis, with 120 of water, in another bottle. The
Schroff proved magnesia to possess antidotal powers in cases of poisoning by arsenic and cobalt, if given early (Medico - Chirurgical Review, i., 1859). Sugar and magnesia mixed together have been found useful (Lancet, ii., 1873, p. 157). Orfila proposed it as an antidote to phosphorus, and there is some but not conclusive evidence in its favor (Medico-Chirurgical Review, i., 1857).
Magnesia being smooth, light, non-irritant, and antacid, makes a good absorbent dusting powder. It has been used for erythema, erysipelas, and similar inflammatory conditions of the skin, and also for atonic ulcers, exposed surfaces, and inflamed wounds.