Antacids are basic compounds mat neutralise hydrochloric acid in the gastric secretions. They are used in the symptomatic management of gastrointestinal disorders associated with gastric hyperacidity such as dyspepsia, gastroesophageal reflux disease, and peptic ulcer disease (see below).
Antacids do not reduce the volume of hydrochloric acid secreted and elevation of the gastric pH may actually promote an increase in acid secretion. However, mis is usually minor and short-lived except after large doses of calcium carbonate. Antacids are normally given between meals and at bedtime when symptoms of gastric hyperacidity usually occur the presence of food in the stomach can prolong the neutralising activity. Some calculate doses as mEq or mmol of acid-neutralising capacity, but the relationship between neutralising capacity and beneficial effect is not straightforward. Other factors, including formulation (liquid preparations are more effective man solids) and duration of action (relatively insoluble antacids are longer acting) are also important.
Aluminium salts tend to produce constipation and to delay gastric emptying, while magnesium salts have the reverse effect a combination of the two may reduce adverse gastrointestinal effects. Another advantage of combined antacid formulations is mat a slow-acting antacid such as aluminium hydroxide may be combined with a more rapidly acting drug such as magnesium hydroxide to improve the onset and duration of effect. Alternatively, complexes containing both aluminium and magnesium may be used, such as almasilate, hydrotalcite, and magaldrate. Other drugs mat may be combined with antacid formulations include simeticone, which acts as a defoaming agent to reduce excess gas in the stomach, and alginates, which form a gel or foam on the surface of the stomach contents thereby impeding reflux and protecting the oesophageal muco-sa from acid attack.
Calcium carbonate and sodium bicarbonate are bom rapidly acting but have disadvantages: calcium carbonate is usually reserved for short-term treatment because of the risks of rebound acid secretion and metabolic alkalosis, while sodium bicarbonate is absorbed and is contra-indicated in patients who must control sodium intake (e.g. in heart failure, hypertension, renal failure, cirrhosis, or pregnancy).
Antacids may interact with numerous other tings, affecting the rate and extent of their absorption, and in some cases their renal elimination. Changes in gastric pH affect the dissolution of other drugs, and together with altered gastric emptying can markedly influence absorption.
Aluminium compounds in particular are noted for their propensity to adsorb other drugs and to form insoluble complexes mat are not absorbed. Antacids mat alter urinary pH will affect renal clearance of drugs mat are weak acids or bases. Several mechanisms may play a part in any particular interaction. Interactions can be minimised by giving antacids and other medication 2 to 3 hours apart. Antacids in mis chapter include aluminium salts, magnesium salts, and calcium carbonate. For sodium bicarbonate.
List of Antacids
Aluminium Hydroxide-Magnesium Carbonate Co-dried Gel
Aluminium Sodium Silicate
Basic Aluminium Carbonate
Dihydroxyaluminum Sodium Carbonate