Gastrointestinal Agents: Antacids


Antacids are substances that neutralize stomach acidity, providing relief from symptoms like heartburn and indigestion. They work by chemically counteracting excess acid produced by the stomach. Commonly taken orally, antacids are used for conditions such as ulcers, acid reflux, and occasional indigestion. While generally safe, they do not address underlying causes. Use of antacids is to give symptomatic relief from pain by neutralizing the excess of hydrochloric acid. Hence, they are useful in treatment of ulcers by reducing pain. In this article we will see antacids, general properties of antacids and combination of antacids.

antacids: acidity
antacids: acidity     source: wikimedia

Ideal properties of antacids

  • It should be insoluble in water and should have fine particle size.
  • It should not produce systemic alkalosis.
  • It should produce its effect gradually and over a longer period of time.
  • It should not have side effects.
  • It should be stable, readily available.
  • It should not evolve large quantities of gas on reacting with hydrochloric acid.
  • It should be non-toxic and should not have any side effects.
  • They should not cause constipation or have any laxative properties.
  • It should inhibit formation of hydrochloric as well as pepsin.

Combination of antacids

Combinations of antacids are often used to provide more comprehensive relief from symptoms of acid reflux, heartburn, and indigestion. Antacids act by neutralization reaction with liberation of carbon dioxide gas. The evolution of gas causes discomfort and distortion of stomach. Hence, antacid preparations contain antiflatulents to avoid discomfort. Some preparations contain simethicone as a surface-active agent to disperse the foam. This effect is helpful in reducing gastro esophageal reflexes. Sometimes alginic acid is used as it is thought to protect against irritating effect of hydrochloric acid during period.

Antacid combinations offer a broader spectrum of relief by addressing different aspects of digestive discomfort. They can be more effective than individual antacids alone.ds of esophageal reflux. Combine antacids with lifestyle modifications (such as avoiding trigger foods and eating smaller, frequent meals) for better management of acid reflux and other digestive issues.

Sodium bicarbonate

Molecular formula: NaHCO3

Molecular weight: 84.01


At industrial level, it is prepared using Brine solution (NaCl). Firstly, NaCl is saturated with ammonia to remove impurities, the solution is filtered and passed through the carbonating tower. Here, it is allowed to react with a current of CO2 and the tower is cooled to enhance precipitation. The precipitate is filtered out and dried. This method cannot produce medicinal grade sodium bicarbonate.

H2O + CO2 — > H2CO3

NH3 + H2CO3 — > NH4HCO3

NaCl + NH4HCO3 — > NaHCO3 + NH4Cl

At the laboratory level, it is prepared by passing CO2 gas through solution of sodium hydroxide. The solution is concentrated to obtain the product. This method produces medicinal grade sodium bicarbonate.

2NaOH + CO2 — > Na2CO3 + H2O

Na2CO3 + H2O + CO2 — > NaHCO3


  • White crystalline powder
  • Saline in taste
  • Soluble in water
  • Insoluble in organic solvents
  • Aqueous solution is alkaline in nature
  • When reacts with mineral acids, produces effervescence


  • Antacid in systemic acidosis.
  • In cases of acute kidney injury (AKI), sodium bicarbonate is used to reverse blood acidity.
  • It may also help slow the progression of chronic kidney disease (CKD) by treating acidosis.
  • Sodium bicarbonate can improve oral health.
  • It may help reduce dental plaque and tooth discoloration.

Dose: 300mg to 2g per day

Assay: It is assayed as base by titrating with standard acid using methyl orange as indicator. It contains 99% to 100.5% of NaHCO3, calculated with reference to dried substance.

Aluminum hydroxide gel

Aluminium hydroxide gel is an aqueous suspension of hydrated aluminium oxide with varying quantities of aluminium carbonate and bicarbonate. It contains not less than 3.5% and not more than 4.4% of aluminium oxide. It may also contain glycerine, sorbitol, saccharine, peppermint oil and preservative like sodium benzoate.


A solution of potash alum is mixed to the hot solution of sodium carbonate with continuous stirring. Then precipitated aluminium hydroxide is filtered, washed thoroughly with hot water. Finally, the precipitated is suspended in water to the required strength and it is homogenised.

3Na2CO3 + 2KAl(SO4)2 + 3H2O — > 3Na2SO4 + K2SO4 + 2Al(OH3) + 3CO2


  • White viscous suspension
  • Small amount of clear liquid may separate on standing

Uses: Antacid


It is assayed for the content of aluminium oxide by complexometric titration with back titrating technique.

Magnesium hydroxide

Molecular formula: Mg(OH)2

Molecular weight: 58.32


It is obtained by evaporating milk of magnesia to dryness. It can also be prepared by treating sea water with sufficient amount of calcium hydroxide to precipitate as magnesium hydroxide.


  • White fine amorphous powder
  • Practically insoluble in water
  • Alkaline in nature
  • Dissolves in dilute mineral acids

Use: Antacid


Antacids are medications commonly used to relieve symptoms of indigestion, heart burn and acid reflux by neutralising stomach acid. They work by raising the pH level of stomach acid, reducing irritation of the oesophagus and stomach lining. Majorly used inorganic antacids are aluminium hydroxide gel, sodium bicarbonate and magnesium hydroxide. Antacids are available over the counter (OTC) in various form such as tablets, liquids and chewable tablet. They are generally safe when used as directed but long-term use can lead to side effects such diarrhoea, constipation and electrolyte imbalances.

Frequently asked questions

What is antacid?

Antacids are substances that neutralize stomach acidity, providing relief from symptoms like heartburn and indigestion.

What are antacids examples?

Some examples of antacids are, sodium bicarbonate, aluminium hydroxide gel, magnesium hydroxide, magnesium carbonate, magnesium trisilicate, etc.

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