Antacids

April 4, 2020 Rima Pharmacology 0

What are antacids?

Antacids are weak bases which on ingestion lower the acidity of gastric contents by neutralizing the acid. Antacids were used as first-line defense against peptic ulcer before introduction of proton-pump inhibitors. Currently antacid use is restricted to relief of mild intermittent gastroesophageal reflux disease (GERD) associated heartburn and for symptomatic, immediate relief in patients with duodenal ulcer, gastritis and reflux esophagitis. As OTC drugs, they are often used for symptomatic treatment of dyspepsia.

Mechanism of action

They reduce the acid reaching the duodenum by neutralizing the acid present in the stomach. They raise the gastric pH to above 4. Some antacids like sodium bicarbonate can raise the pH above 7. The relative effectiveness of antacid is measured in milliequivalent of acid neutralizing capacity.

Uses of antacid

Antacids have therapeutic use for the following:

  • Heartburn symptoms in GERD
  • Duodenal and gastric ulcer
  • Stress gastritis
  • Pancreatic insufficiency
  • Biliary reflux
  • Constipation
  • Urine alkalization
  • Phosphate binding in chronic renal failure

Classification of antacid

  • Non-systemic antacids: e.g. Aluminium hydroxide gel, Calcium carbonate, Magnesium hydroxide,

Magnesium trisilicate

  • Systemic antacids: e.g. Sodium bicarbonate

1. Non-systemic antacids: They are water insoluble agents in which base may be hydroxide, trisilicate or carbonate combined with cation such as Mg++, Ca++ or Al+++. During neutralization of acid, the cation forms chloride with Cl– of HCl for e.g. magnesium chloride. In small intestine, this chloride salt reacts with bicarbonate from intestinal juices to form original salt (hydroxide or carbonate). Thus, there is no net gain or loss of H+ or HCO3– ions and hence no chances of systemic alkalosis.

Aluminium hydroxide: It is available in the form of tablets, powder or as white, colloidal, viscous suspension. Each ml of 4% gel suspension neutralizes 1.2 -2.5 mEq of acid. It raises gastric pH to 4 and also inhibits pepsin activity. It is complexed with sulfated polysaccharide sucrose to form sucralfate. Sucralfate doesn’t have buffering action on acid and doesn’t affect pepsin secretion. It prevents acute mucosal damage and heal chronic ulcer by reducing access to pepsin and acid. Its common adverse effect is constipation which can be minimized by combining it with magnesium trisilicate.

Preparations and dosage: (i) Aluminium hydroxide gel 4-8 ml every 2-4 hours. (ii) Aluminium hydroxide 0.5 g tablets; 1-2 tablets to be chewed qid (4 times a day).

Magnesium trisilicate: It is fine, white, tasteless powder insoluble in water. It reacts with acid to form hydrated silicon dioxide. Its neutralizing action is slow in onset but lasts for long time. 1 gm of magnesium trisilicate neutralizes about 9-11 mEq of gastric acid. The most common adverse effect is diarrhea which is countered by combining with aluminium hydroxide. In patients with impaired renal function, it may produce CNS depression.

Preparation and Dose: 2-4 g every 1-4 hours. The tablet should be chewed before it is swallowed. The drug can also be used in powder form.

Magnesium oxide and hydroxide: Magnesium oxide on contact with water, is converted to magnesium hydroxide which react with gastric acid. It has quick onset of action and act for prolonged period. One gm of magnesium oxide neutralizes 50 mEq of the acid. Magnesium hydroxide is available as ‘Milk of magnesia’ which is more palatable compared to other magnesium containing formulations. It contains 7-8.5 % of magnesium hydroxide. One ml of milk of magnesia neutralizes 2.7 mEq of acid. It can cause mild diarrhea.

Dose: Milk of magnesia as an antacid 4 ml; as a laxative 15 ml.

Calcium carbonate: It reacts with gastric acid to form calcium chloride. It has quick onset of action, acts for prolonged period and is inexpensive. It has high neutralizing capacity; 1 gm calcium carbonate can neutralize 21 mEq of the acid. Some adverse effects of calcium carbonate are constipation, prolonged therapy may lead to hypercalcemia, hyperphosphatemia and renal calcinosis.

Dose: Powdered precipitated chalk or tablet containing 1 g of calcium carbonate. Dose: 2-4 g.

2. Systemic antacids: Systemic antacids like sodium bicarbonate is completely absorbable antacid. Sodium bicarbonate is a white, water soluble antacid which react with gastric acid to from sodium chloride, carbon dioxide and water. The removal of carbon dioxide formed during neutralization gives the sense of relief from abdominal discomfort. It is an effective and rapidly acting antacid but have short duration of action. 1 gm of drug neutralizes 12 mEq of acid.

It may cause systemic alkalosis and edema. Some other side effects are flatulence, feeling of fullness, nausea and exacerbation of esophageal reflux.

Dose– 1-5 gm in water, repeated as required.

                   Figure- Mechanism of action of systemic and non-systemic antacids

 Antacid therapy

Antacids are most commonly abused drugs. Their indiscriminate use can mask the warning symptoms of serious disease like cancer of stomach. So, antacids should be used properly and in adequate doses. Following points should be considered while using antacids:

  • They should be prescribed in optimal dose at 1 and 3 hours after each meal, at bedtime and as needed for pain.
  • The quantity of an antacid required to produce optimal buffering may differ from individual to individual.
  • Antacid tablets are more convenient than liquid to use but are less effective buffers. So liquid formulations are preferred. If tablets are used, they should be chewed thoroughly for maximum effect.
  • Antacids may interfere with absorption of other drugs (thyroid hormones, allopurinol and imidazole antifungals).

While choosing antacids, generally systemic antacids should be avoided. Cost should be considered. Magnesium trisilicate is more effective than aluminium hydroxide. Combination of Mg++ (rapidly reacting) and Al+++ (slowly reacting) hydroxides provide antacid with balanced and sustained neutralizing capacity. Magaldrate is hydroxymagnesium aluminate complex that is converted to Mg (OH)2 and Al (OH)3 in gastric acid which are absorbed poorly and provide a sustained antacid effect.

Contraindications

The absolute contraindication is hypersensitivity to any component of the formulation. It should also be administered with caution in patients with:

  • Renal failure
  • Heart failure
  • Edema
  • Low sodium diets
  • Uremia
  • Renal calculus
  • Achlorhydria
  • GI hemorrhages

References

  • Sontag SJ. The medical management of reflux esophagitis. Role of antacids and acid inhibition. Clin. North Am. 1990 Sep;19(3):683-712.
  • Maton PN, Burton ME. Antacids revisited: a review of their clinical pharmacology and recommended therapeutic use. Drugs. 1999 Jun;57(6):855-70.
  • Weberg R, Berstad K, Berstad A. Acute effects of antacids on gastric juice components in duodenal ulcer patients. J. Clin. Invest. 1990 Oct;20(5):511-5.
  • Tarnawski A, Tanoue K, Santos AM, Sarfeh IJ. Cellular and molecular mechanisms of gastric ulcer healing. Is the quality of mucosal scar affected by treatment? J. Gastroenterol. Suppl. 1995; 210: 9-14.
  • Soll AH, Weinstein WM, Kurata J, McCarthy D. Nonsteroidal anti-inflammatory drugs and peptic ulcer disease. Intern. Med. 1991 Feb 15;114(4):307-19.
  • Schucker JJ, Ward KE. Hyperphosphatemia and phosphate binders. Am J Health Syst Pharm. 2005 Nov 15;62(22):2355-61.
  • R. Jacyna, E. J. Boyd, and K. G. Wormsley. Comparative study of four antacids. Postgrad Med J. 1984 Sep; 60(707): 592–596.
  • Arnold Berstad & Ragnar Weberg. Antacids in the Treatment of Gastroduodenal Ulcer. Scandinavian Journal of Gastroenterology. 21:4, 385-391.
  • Pharmacology and pharmacotherapeutics. Page no- 636-639.
  • Goodman and Gillman Manual of Pharmacology and Therapeutics. Page no- 627-628.
  • A textbook of clinical pharmacology and therapeutics. Page no- 249-250.