Magnesium ions react with gastric acid to produce magnesium chloride. Magnesium carbonate s crystal structure is responsible for its slower onset of action. Magnesium trisilicate is a relatively weak antacid that is poorly soluble and produces the slowest onset of action and longest duration of action of all the magnesium preparations. Magnesium compounds alter gastrointestinal motility and secretion and can result in a dose-limiting diarrhea. Alkalization of gastric contents stimulates gastrin to increase gastric motility.
Osmotic diarrhea results from poorly absorbed, unreacted magnesium salts in the small intestine.
In contrast, aluminum relaxes gastric smooth muscle, delays gastric emptying, and has a constipating effect. Therefore, magnesium salts are often combined with aluminum salts in antacids to balance their effects. Magnesium laxatives are indicated for acute bowel evacuation before diagnostic evaluation, for treatment of severe constipation from chronic opioid or laxative abuse, or for occasional use in healthy adults. Onset of laxative effect is observed within 30 minutes to 3 hours. Magnesium s mechanism of action is as an oral saline laxative and cathartic primarily by its osmotic effect in the colon.