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.
Most of the soluble salts of iron have an inky astringent taste, and by continued use, stain the teeth and mouth of a dark color (tannate of iron). Compounds with a mineral acid exert a local astringent action on the mouth and stomach, and if the dose be small and. diluted, may improve the tone and the functional power of the gastric membrane: but these, or any other preparation, if given in undue quantity, may irritate, and cause indigestion (from lessened secretion), with sense of weight, nausea, or diarrhoea.
Quevenne experimented with gastric fluid withdrawn through a fistula from the stomach of dogs, and judged of the effects of iron on digestion by the precipitates of peptones obtained from the fluid at certain periods after a meal. There was less precipitate when the juice was acid than when partly neutralized, but he concluded that various forms of iron, given with food, improved the character and amount of the precipitate: they did not increase the proportion of pepsine, nor alter the duration of the digestive process, but were quite readily absorbed, and the dogs thrived and gained flesh under their use. On the other hand, when given without food, and especially in the metallic form, iron did not stimulate the formation of sufficient secretion to dissolve itself, but acted as a foreign body, and impaired digestion: 10 to 20 gr. of reduced iron would cause diarrhoea, hence a reason for the ordinary rule of ordering iron preparations at the time of a meal, and in small doses (2 to 3 gr.). The sulphate and chloride of iron have sometimes, by mistake or for criminal purposes, been taken in large quantities (1 oz. and upward), and have caused violent pain and vomiting, with other symptoms of irritant poisoning, and gastro-enteritis, but have rarely proved fatal (Taylor).