Category:Antacids

An antacid is a medicine taken to neutralise gastric acid that has already been secreted, providing rapid (within minutes) and short-lived (typically 30 to 60 minutes after a meal) symptomatic relief of heartburn, acid reflux, and acid-related dyspepsia. The category sits in contrast to the antisecretory agents (which reduce acid secretion rather than neutralise it) and is composed of inorganic bases of sodium, magnesium, aluminium, and calcium, used singly or in combination.

The antacid category is among the oldest in clinical pharmacology, predating systematic medicine entirely. The use of chalk (calcium carbonate) and of crushed coral for stomach upset is documented in classical antiquity. The German pharmacist Friedrich Hoffmann popularised sodium bicarbonate in the eighteenth century (Hoffmann's drops). The 1890s introduction of aluminium hydroxide gel and the early-twentieth-century introduction of magnesium hydroxide ("milk of magnesia", Charles Henry Phillips 1872) gave the antacid pharmacopoeia its three classical components, which have been combined in various ratios in proprietary preparations ever since.

The contemporary clinical place of antacids is symptomatic relief rather than acid-peptic disease cure. The introduction of effective antisecretory therapy (the H2 antagonists from 1976, the proton pump inhibitors from 1988; described under antisecretory agents and anti-ulcer agents) substantially reduced the role of antacids in peptic-ulcer treatment, where they had been a 24-hour-around-the-clock burden requiring frequent dosing. Antacids remain in use for occasional symptomatic GERD, for symptom-bridging during PPI initiation (where the PPI takes 3-5 days to reach steady-state effect), for selected pregnancy-associated heartburn (where the avoidance of systemic medicine drives interest in non-absorbed antacids), and for the rare patient who does not require long-term acid suppression.

The pharmacology of the four principal antacid components differs in clinically important ways. Sodium bicarbonate (NaHCO3, as Alka-Seltzer or in compounded preparations) provides rapid neutralisation but produces systemic alkalosis on chronic high-dose use, contributes a substantial sodium load (problematic for hypertensive and heart-failure patients), and produces CO2 gas on neutralisation that may exacerbate distention. Calcium carbonate (Tums, Rolaids, Mylanta Calci-Tabs) provides rapid neutralisation and the added benefit of calcium supplementation, but produces rebound acid hypersecretion (the "calcium-stimulated gastrin" phenomenon) on chronic high-dose use; the milk-alkali syndrome, an iatrogenic hypercalcemic alkalosis from concurrent high-dose calcium carbonate and dairy, was a clinical problem in the era of intensive ulcer-disease antacid therapy and remains a possible complication of inappropriate self-medication. Aluminium hydroxide (Amphojel) provides somewhat slower but more sustained neutralisation, binds intestinal phosphate (the basis of its use in chronic-kidney-disease hyperphosphatemia, where the long-term aluminium-toxicity concern has largely retired it in favour of non-aluminium phosphate binders), and produces constipation. Magnesium hydroxide (Milk of Magnesia) provides intermediate neutralisation, produces diarrhoea (and is used at higher doses as an osmotic laxative; cross-listed under osmotic laxatives), and is contraindicated in significant renal impairment because of magnesium accumulation.

The combination antacids (Maalox: aluminium hydroxide + magnesium hydroxide ± simethicone; Mylanta: same; Gaviscon: aluminium hydroxide or sodium bicarbonate + magnesium carbonate + alginate, the alginate forming a viscous raft that floats on gastric contents and inhibits regurgitation; Tums: calcium carbonate alone) are designed to balance the constipating and diarrhoeal effects of the components and to provide either rapid (calcium carbonate, sodium bicarbonate) or sustained (aluminium hydroxide) neutralisation as the clinical pattern requires. The alginate-containing preparations (Gaviscon family) provide a mechanically distinct anti-reflux mechanism beyond simple neutralisation.

The interaction profile of the antacids deserves mention. Aluminium-containing and magnesium-containing antacids reduce the absorption of orally administered tetracycline, fluoroquinolone, and azithromycin antibacterials (chelation with divalent cations), of levothyroxine, of bisphosphonates, of iron supplements, and of several other medicines; the standard recommendation is a 2-to-4-hour separation between antacid and any of these. Sodium-containing antacids produce sodium loading; calcium-containing antacids interact with bisphosphonate absorption. Antacids that neutralise gastric acid also affect the dissolution and absorption of medicines that require gastric acidity for absorption (notably the azole antifungals ketoconazole and itraconazole, the iron supplements at lower elemental iron content).

By active component:

• Sodium-based:
  • Sodium bicarbonate (Alka-Seltzer; compounded; Bromo-Seltzer historical)
• Calcium-based:
  • Calcium carbonate (Tums, Rolaids, Caltrate dual function as calcium supplement; cross-listed under calcium supplements)
  • Calcium acetate (used principally as a phosphate binder in chronic kidney disease)
• Aluminium-based:
  • Aluminium hydroxide (Amphojel, Alternagel; standalone use largely retired)
  • Aluminium carbonate (Basaljel)
• Magnesium-based:
  • Magnesium hydroxide (Milk of Magnesia, Phillips')
  • Magnesium oxide
  • Magnesium carbonate
  • Magnesium trisilicate
• Combination antacids:
  • Aluminium hydroxide + magnesium hydroxide ± simethicone: Maalox, Mylanta, Riopan
  • Calcium carbonate + magnesium hydroxide: Mylanta Calci-Tabs, Rolaids Extra Strength
  • Alginate-containing rafts: Gaviscon (multiple formulations across regions; aluminium hydroxide or sodium bicarbonate + magnesium carbonate + sodium alginate)
• Alginate-only "rafting" antacid:
  • Sodium alginate or potassium alginate preparations (Gaviscon Advance, Gaviscon Liquid; rafting mechanism distinct from neutralisation)

The boundary of this category is "medicine that neutralises gastric acid that has already been secreted." The antisecretory agents reduce acid secretion rather than neutralise it and are listed separately. The mucosal protectants (sucralfate, bismuth) act on the gastric mucosa rather than on the acid and are listed separately. The medicines for functional dyspepsia not responsive to acid management (prokinetics, the SSRIs at low dose, tricyclics) are listed under their primary class. The combined antacid-and-pain-relief OTC preparations (Goody's Headache Powder, BC Powder, Pepto-Bismol's salicylate component, Alka-Seltzer's aspirin-plus-sodium-bicarbonate) are cross-listed at the analgesic categories for the analgesic component.

This category page is an encyclopedia article about its subject. The actual index of medicines belonging to the category is generated automatically by the wiki engine, from category-membership declarations on the individual medicine pages, and appears at the foot of the page below the references.