Antacids have been used for centuries in the treatment of patientswith
dyspepsia and acid-peptic disorders. They were the mainstay of treatment for
acid-peptic disorders until the advent of H2−receptor antagonists and proton
pump inhibitors. They continue to be used commonly by patients as
nonprescription remedies for the treatment of intermittent heartburn and
are weak bases that react with gastric hydrochloric acid to form a salt and
water. Their principal mechanism of action is reduc-tion of intragastric
acidity. After a meal, approximately 45 mEq/h of hydrochloric acid is secreted.
A single dose of 156 mEq of antacid given 1 hour after a meal effectively
neutralizes gastric acid for up to 2 hours. However, the acid-neutralization
capacity among different proprietary formulations of antacids is highly
variable, depending on their rate of dissolution (tablet versus liquid), water
solubility, rate of reaction with acid, and rate of gastric emptying.
Sodium bicarbonate (eg, baking soda, Alka Seltzer) reacts rap-idly with
hydrochloric acid (HCL) to produce carbon dioxide and sodium chloride.
Formation of carbon dioxide results in gastric distention and belching.
Unreacted alkali is readily absorbed, poten-tially causing metabolic alkalosis
when given in high doses or to patients with renal insufficiency. Sodium
chloride absorption may exacerbate fluid retention in patients with heart
failure, hyperten-sion, and renal insufficiency. Calcium carbonate (eg, Tums, Os-Cal) is less soluble and reacts
more slowly than sodium bicarbonate with HCl to form carbon dioxide and calcium
chloride (CaCl2). Like sodium
bicarbonate, calcium carbonate may cause belching or meta-bolic alkalosis.
Calcium carbonate is used for a number of other indications apart from its antacid
properties . Excessive doses of either sodium bicarbonate or calcium carbonate
with calcium-containing dairy products can lead to hypercalcemia, renal
insufficiency, and metabolic alkalosis (milk-alkali syndrome).
containing magnesium hydroxide or aluminumhydroxide react slowly with HCl
to form magnesium chloride oraluminum chloride and water. Because no gas is
generated, belching does not occur. Metabolic alkalosis is also uncommon
because of the efficiency of the neutralization reaction. Because unabsorbed
magne-sium salts may cause an osmotic diarrhea and aluminum salts may cause
constipation, these agents are commonly administered together in proprietary
formulations (eg, Gelusil, Maalox, Mylanta) to mini-mize the impact on bowel
function. Both magnesium and aluminum are absorbed and excreted by the kidneys.
Hence, patients with renal insufficiency should not take these agents
antacids may affect the absorption of other medications by binding the drug
(reducing its absorption) or by increasing intra-gastric pH so that the drug’s
dissolution or solubility (especially weakly basic or acidic drugs) is altered.
Therefore, antacids should not be given within 2 hours of doses of
tetracyclines, fluoroquino-lones, itraconazole, and iron.