The two most common ingredients in over-the-counter antacid combination products are a magnesium salt and an aluminum salt. The rationale for this particular combination is that: (LQ)
|A||Al salts counteract the gastric mucosal-irritating effects of Mg salts|
|B||Al salts require activation by an Mg-dependent enzyme|
|C||Mg salts cause a diuresis that helps reduce systemic accumulation of the Al salt by increasing renal Al excretion|
|D||Mg salts tend to cause a laxative effect that counteracts the tendency of an Al salt to cause constipation|
a. Magnesium salts used alone tend to cause a laxative effect. (Indeed, at dosages higher than those used for acid neutralization, magnesium salts are used for their laxative or cathartic effects.) Aluminum (and calcium) antacids, given alone, tend to cause constipation. Combining a magnesium salt with an aluminum (and/or calcium antacid) is an often successful approach to minimizing antacid-induced changes of net gut motility
b. Magnesium salts do not potentiate the antisecretory actions of alluminium containing or any other antacid. Indeed, none of the antacids inhibit gastric acid secretion (i.e., they don’t have an antisecretory effect to begin with). They merely neutralize acid that has already been secreted (Mg, Ca, sodium bicarbonate) or adsorb acid and pepsins (most of the aluminum compounds).