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{{wiktionary|antacid}}
[[Image:Antacid-L478.jpg|thumb|A bottle of antacid tablets]]
An '''antacid''' is any substance, generally a base, which counteracts [[gastric acid|stomach acidity]]. In other words, antacids are stomach acid [[neutralization|neutralizers]].

==Action mechanism==
Antacids perform a neutralization reaction, i.e. they [[buffering agent|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 and nociception|pain]] to the central nervous system. This happens when these nerves are exposed, as in [[peptic ulcer]]s. 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.

==Indications==
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 ulcer]]s may require [[H2-receptor antagonist]]s or [[proton pump inhibitor]]s.

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

==Side effects==
* ''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.

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

==Problems with reduced stomach acidity==
Reduced stomach acidity may result in an impaired ability to [[Digestion|digest]] and absorb certain [[nutrient]]s, such as [[iron]] and the [[Vitamin B|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).

==Drug names==
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)


{{Major Drug Groups}}
{{Antacids}}
[[Category:Gastrointestinal system drugs]]

[[ca:Antiàcid]]
[[de:Antazidum]]
[[es:Antiácido]]
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[[pt:Antiácido]]
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[[sl:Antacid]]
[[sr:Антацид]]
[[sh:Antacid]]
[[fi:Antasidi]]
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[[th:ยาลดกรด]]
[[tr:Antiasitler]]