This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
Stomach and Intestines. - Magnesia and magnesium carbonate are antacid, acting in many ways like the potassium and sodium alkalies. Carbon dioxide is given off, if the carbonate has been given, and is sedative to the stomach. They are both decomposed by the gastric juice, magnesium chloride, lactate and bicarbonate being formed. These salts, or the sulphate, if that has been taken, act in the intestine, as typical saline purgatives. The sulphate is most powerful. The mode of action of this group of purgatives has been discussed on p. 93.
Like other alkaline remedies, these magnesium salts increase the alkalinity of the blood, alkalize the urine, help to keep uric acid in solution, and are diuretic. But their action on the blood and urine is feebler than that of potassium and sodium salts, for they are with difficulty absorbed. Large doses injected into the blood of animals are toxic, killing by their action on the heart.
Stomach. - Magnesia and the carbonate are mild alkaline remedies, and may be used in the same class of cases as other alkalies. They form insoluble compounds with mineral acids, oxalic acid, and mercury, arsenic and copper salts. By alkalizing the gastric contents they hinder the absorption of alkaloids. They are, therefore, antidotes to all these substances; the objection to them is their bulk. Magnesia is to be preferred, as the carbonate gives off carbon dioxide gas. They must be freely given. The sulphate is an antidote to lead and barium salts, forming insoluble sulphates.
The magnesium salts are very common purgatives. Magnesia, the carbonate, and the citrate are excellent for children. The sulphate is one of our best saline purgatives. It is very largely used, especially for the varieties of constipation that are associated with hepatic disorder, gout or excessive uric acid. Its use is then spread over some time, and it may conveniently be taken as one of the mineral waters which contain it and sodium sulphate (see p. 144). A concentrated solution, causing as it does an increased secretion of intestinal fluid, is a useful purge for dropsy or ascites. It is useful with glycerin in concentrated enema for thorough cleansing of the bowels before surgical operations (glycerin, 1 oz.; 30. c.c., in a saturated solution of magnesium sulphate, in hot water 3 oz.; 90. c.c., which is allowed to cool). It can also be used hypodermatically in dose of 3 gr.; .20 gm., which frequently will cause a watery evacuation. In operations during which the abdomen is opened, the subsequent intestinal paralysis can be prevented from causing constipation by injecting into the small intestine through a cannula one ounce; 30. c.c. of a saturated solution of magnesium sulphate. The wound in the bowel should be closed by a Lembert stitch.
So little of these salts is absorbed that they are only to be given for their alkaline effects on the blood and urine in those cases of gout and uric acid gravel in which potassium or sodium salts cannot be borne.