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A bottle of antacid tablets

An antacid is any substance, generally a base, which counteracts stomach acidity. In other words, antacids are stomach acid neutralizers.

Antacids perform a neutralization reaction, i.e. they buffer gastric acid, raising the pH to reduce acidity in the stomach. When gastric hydrochloric acid reaches the nerves in the gastrointestinal mucosa, they signal pain to the central nervous system. This happens when these nerves are exposed, as in peptic ulcers. The gastric acid may also reach ulcers in the esophagus or the duodenum.

Other mechanisms may contribute, such as the effect of aluminum ions inhibiting smooth muscle cell contraction and delaying gastric emptying.

Antacids are taken by mouth to relieve heartburn, the major symptom of gastroesophageal reflux disease, or acid indigestion. Treatment with antacids alone is symptomatic and only justified for minor symptoms. Peptic ulcers may require H2-receptor antagonists or proton pump inhibitors.

The utility of many combinations of antacids is not clear, although the combination of magnesium and aluminum salts may prevent alteration of bowel habits.

• Aluminum hydroxide: may lead to the formation of insoluble aluminum-phosphate-complexes, with a risk for hypophosphatemia and osteomalacia. Although aluminum has a low gastrointestinal absorption, accumulation may occur in the presence of renal insufficiency. Aluminum-containing drugs may cause constipation.
• Magnesium hydroxide: has laxative properties. Magnesium may accumulate in patients with renal failure leading to hypermagnesemia, with cardiovascular and neurological complications. See Milk of magnesia.
• Carbonate: regular high doses may cause alkalosis, which in turn may result in altered excretion of other drugs, and kidney stones. A chemical reaction between the carbonate and hydrochloric acid may produce carbon dioxide gas. This causes gastric distension which may not be well tolerated.
• Calcium: compounds containing calcium may increase calcium output in the urine, which might be associated to renal stones. Calcium salts may cause Constipation.
• Sodium: increased intake of sodium may be deleterious for arterial hypertension, heart failure and many renal diseases.

Altered pH or complex formation may alter the bioavailability of other drugs, such as tetracycline. Urinary excretion of certain drugs may also be affected.

Reduced stomach acidity may result in an impaired ability to digest and absorb certain nutrients, such as iron and the B vitamins. Since the low pH of the stomach normally kills ingested bacteria, antacids increase the vulnerability to infection. It could also result in reduced bioavailability of some drugs.For example,the bioavailability of ketoconazole(antifungal),is reduced at high intragastric pH.(low acid content).

Examples of antacids (brand names may vary in different countries).

• Aluminum hydroxide (Amphojel^®, AlternaGEL^®)
• Magnesium hydroxide (Phillips’^® Milk of Magnesia)
• Aluminum hydroxide and magnesium hydroxide (Maalox^®, Mylanta^®)
• Aluminum carbonate gel (Basaljel^®)
• Calcium carbonate (Alcalak^®, Calcium Rich TUMS^®, Quick-Eze^®, Rennie^®, Titralac^®, Rolaids^®)
• Sodium bicarbonate (Bicarbonate of soda, Alka-Seltzer^®)
• Hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆ · 4(H₂O); Talcid^®)
• Bismuth subsalicylate (Pepto-Bismol)
• Magaldrate + Simethicone (Pepsil)