Antacids - Agents that Reduce Intragastric Acidity

Antacids

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 dyspepsia.

Antacids 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).

Formulations 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 long-term.

All 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.