Antacid

Antacids are alkaline substances designed to neutralise excess gastric acid and thereby relieve symptoms such as heartburn, indigestion and general stomach discomfort. They are among the most widely used over-the-counter remedies for mild, occasional gastrointestinal symptoms and have long played a significant role in symptomatic management of acid-related conditions.

Medical Uses

Antacids act locally within the stomach and provide rapid, short-term relief from heartburn and indigestion, which are common features of gastro-oesophageal reflux disease. Their use is generally restricted to mild and infrequent symptoms, as they do not address underlying causes of acid overproduction.
Some preparations have additional therapeutic roles. Certain formulations are used for constipation or diarrhoea, hyperphosphataemia, or urinary alkalisation. Selected non-particulate antacids, such as trisodium citrate, increase gastric pH without significant change to gastric volume and may be used in preoperative protocols when administered shortly before surgery. Antacids can also serve as adjuncts to pancreatic enzyme replacement therapy in patients with exocrine pancreatic insufficiency.

Side Effects

Antacid tolerability varies with their chemical composition. Effervescent preparations often contain substantial sodium amounts and have been associated with increased cardiovascular risk in susceptible individuals. Magnesium-containing antacids may cause diarrhoea, whereas calcium- or aluminium-containing preparations may lead to constipation. Long-term use of aluminium-based antacids has been linked to increased risk of osteoporosis because aluminium can interfere with mineral absorption and bone metabolism.
Some studies have suggested the possibility of “acid rebound” following high antacid intake, although the clinical relevance of this effect remains uncertain.

Properties and Mechanism of Action

The stomach produces hydrochloric acid to aid digestion, but excessive acid can damage the protective mucous lining, resulting in irritation or contributing to reflux symptoms. Antacids contain alkaline ions that neutralise stomach acid by reacting with hydronium ions to form water, carbon dioxide and corresponding salts. This chemical neutralisation increases gastric pH, reduces irritation of the mucosa and diminishes the activity of pepsin, an enzyme that can aggravate oesophageal damage during reflux.
Antacids differ from other acid-modifying drugs. They do not inhibit acid secretion and therefore act more quickly but for a shorter duration than H₂-receptor antagonists or proton pump inhibitors. They also have no effect on Helicobacter pylori, the bacterium responsible for most peptic ulcers.

Types of Antacids

Antacids are broadly classified into two groups based on their solubility and systemic absorption:

• Systemic (absorbable) antacids: These compounds dissolve in water and can be absorbed into the bloodstream, as in sodium bicarbonate or sodium citrate.
• Non-systemic (non-absorbable) antacids: These remain within the gastrointestinal tract and act locally. Examples include aluminium hydroxide, magnesium carbonate and calcium carbonate.

Drug Interactions

Antacids interact with several orally administered medicines. Metal ions in antacids can bind to drugs such as fluoroquinolone or tetracycline antibiotics, iron supplements and certain antifungals, reducing their absorption. Increased gastric pH may also alter drug solubility: absorption of weak acids may increase, while that of weak bases may decrease. Urinary alkalisation caused by antacids can further modify drug excretion patterns by enhancing elimination of weak acids and reducing that of weak bases.
A generally recommended strategy is to separate antacid dosing from interacting medicines by at least two hours, although evidence for effectiveness varies, especially for drugs affected by changes in urinary pH. Concerns have also been raised that antacids may compromise the stability of delayed-release tablets by increasing gastric pH, potentially causing premature dissolution of pH-dependent coatings.

Formulations

Antacids are available in multiple formulations suited to patient preference and symptom profile.
LiquidsLiquid antacids, often based on magnesium hydroxide or magnesium–aluminium combinations, may act more quickly than solid forms but may have a shorter duration of effect.
Chewable TabletsThese tablets commonly contain carbonate or hydroxide salts that react directly with gastric acid. Typical ingredients include calcium carbonate, magnesium carbonate, aluminium hydroxide and sodium bicarbonate. Well-known brands include Tums, Gaviscon chewable tablets and Maalox.
Effervescent TabletsDesigned to dissolve in water and release carbon dioxide, effervescent antacids frequently use citric acid combined with sodium bicarbonate. Some products contain aspirin, which may irritate the stomach lining. Popular brands include Alka-Seltzer, Gaviscon, Eno and Gelusil.
Antacid formulations may also include simethicone to reduce flatulence or alginic acid to form a foamy barrier that floats on gastric contents, reducing reflux of acid into the oesophagus.