Antacids are weak bases that react with gastric acid to form water and a salt to diminish gastric acidity. Because pepsin (a proteolytic enzyme) is inactive at a pH greater than 4, antacids also reduce pepsin activity.
Antacids are used for symptomatic relief of peptic ulcer disease and GERD, and they may also promote healing of duodenal ulcers. They should be administered after meals for maximum effectiveness.
[Note: Calcium carbonate preparations are also used as calcium supplements for the treatment of osteoporosis.]
Antacid products vary widely in their chemical composition, acid-neutralizing capacity, sodium content, palatability, and price. The efficacy of an antacid depends on its capacity to neutralize gastric HCl and on whether the stomach is full or empty (food delays stomach emptying allowing more time for the antacid to react). Commonly used antacids are combinations of salts of aluminum and magnesium, such as aluminum hydroxide and magnesium hydroxide [Mg(OH)2]. Calcium carbonate [CaCO3] reacts with HCl to form CO2 and CaCl2 and is also a commonly used preparation. Systemic absorption of sodium bicarbonate [NaHCO3] can produce transient metabolic alkalosis. Therefore, this antacid is not recommended for long-term use.
Aluminum hydroxide tends to cause constipation, whereas magnesium hydroxide tends to produce diarrhea. Preparations that combine these agents aid in normalizing bowel function. Absorption of the cations from antacids (Mg2+, Al3+, Ca2+) is usually not a problem in patients with normal renal function; however, accumulation and adverse effects may occur in patients with renal impairment.