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.
Some observers have attributed to magnesia an alterative action, and Grange, Bouchardat, and others state that its habitual use, as in drinking water, will cause goitre. Some support is given to this idea by the fact that enlargement of the thyroid gland in mice has followed after mixing magnesia with their food (Gubler), but on the other hand, many waters from goitrous districts have been analyzed without finding in them a trace of magnesia (Medico-Chirurgical Review, i., 1862, p. 512).
Jolyet and Cahours report magnesian sulphate to be the most toxic of neutral purgative salts, 30 to 90 gr. having caused sudden death in dogs, when injected into the veins. Vulpian noted abolition of voluntary and reflex movements in a frog poisoned by the salt, and its effect has been compared to that of curare, but this comparison requires further support before it can be accepted (Archives de Physiol., Few, 1869).
Absorbent substances, such as charcoal and manganese, aid the mechanical effect of magnesia in powder. Its purgative effects are aided by acids, by the sulphate and citrate of magnesia, and other neutral salts. It is usual to combine the sulphate and carbonate in a mixture, but unless care be exercised they are liable to form lumps which are not readily soluble. The analogues of sulphate of magnesia are the sulphates, phosphates, tartrates, sulphovinates of potash and soda, and the chlorides of sodium and magnesium. Water, cold, and refrigerants generally are other adjuvants of its action. Dr. Laycock found quinine aid the purgative effect of magnesia sulphate, 1 gr. of quinine with only 1 scruple of the salt, given every three or four hours, acting as well as much larger doses given without the tonic: he supposed this to depend upon improvement of nerve-power (Medical Times, i., 1863, p. 54).
Acids given with magnesia destroy its absorbent powers, though increasing purgation; on the other hand, alkalies antagonize its purgative effects by neutralizing gastric acidity. Alcohol, aromatics, and opium lessen its anti-febrile and depletory effects. With regard to opium, Buchheim and Wagner observed that if it be brought in contact with mucous membrane before the saline, no increased flow of liquid occurs, but liquid is absorbed from the membrane: they concluded that opium favored the absorption of the salt, but we hold rather that it acted like Moreau's section of the nerve-supply- narcotizing the terminals, dulling the sense of irritation, and so preventing a flow of liquid toward the part - while absorption from it went on as usual (Gubler).